For chocolate crumbs: heat the oven to 250 degrees fahrenheit. In a microwave, gently melt butter on a med/low setting for 30-60 seconds. Let it cool until it not hot to the touch.
Combine the flour, cornstarch, valrhona cocoa powder, sugar and salt in a bowl. With a spoon, mix all dry ingredients until evenly dispersed.
Add the melted butter and stir until the mixture starts to come together in small clusters.
Spread the clusters on a parchment or silpat lined half sheet pan and bake at 250F for 35 minutes. The crumbs should be slightly moist to the touch at that point; they will harden as they cool.
Let the crumbs cool completely before using in a recipe or eating. In an airtight container, crumbs will keep fresh for 1 week in the pantry or 1 month in the fridge or freezer.
For fudge sauce: combine the chocolate, cocoa powder and salt in a medium mixing bowl and set it to the side.
Combine the sugar, glucose and heavy cream in a heavy bottomed saucepan and bring the mixture to a boil over high heat. The moment it boils, pour it into the bowl holding the chocolate. Let the mixing bowl sit untouched. For one full minute, then begin to slowly, slowly mix with a whisk.
Increase the vigor of your whisking every 30 seconds, until the mixture is glossy and silky smooth. This will take 1-3 minutes, depending on your speed and strength. You can sauce the gelato cake at will from this point on or store the fudge sauce in an airtight container in the refrigerator for up to 2 weeks. (Bring to warm temperature in the microwave or gently on the stove before using for gelato cake.)
For chocolate cake: heat the oven to 325 degrees fahrenheit. In a stand mixer with the paddle attachment, cream the butter and sugar on medium high for 2 to 3 minutes, until fluffy and pale yellow in color. Scrape down the sides of the mixing bowl with a spatula.
On a low speed, add the eggs to incorporate. Increase the mixer speed back up to medium high and paddle the mixture for 3 minutes until it is homogenous and fluffy. There should be no streaks of fat or liquid implying that the mixture is separated and not fully incorporated. Stop the mixer and scrape down the sides of the bowl.
On a low speed, stream in the oil, buttermilk and vanilla extract together. Increase the mixer speed back up to a medium high and paddle the mixture for 3-5 minutes until the mixture is practically white in color, twice the size of your original butter and sugar fluffy mixture, and completely homogenous. There should be no streaks of fat or liquid. Stop the mixer and scrape down the sides of the bowl.
On a very low speed, add the flour, cocoa powder, baking powder, and salt. Mix for 45 to 60 seconds, just until your cake batter comes together. Scrape down the sides of the bowl with a spatula and mix on the same low speed for another 45 seconds to ensure that all the little lumps of cocoa powder and cake flour are incorporated and hydrated within the cake batter.
With cooking spray, spray and line a quarter sheet pan with parchment or a silpat and, using a spatula to help you, evenly spread the entire contents of your mixing bowl into the sheet pan. Give the bottom of your sheet pan a bang on the countertop to even out the layer.
Bake the cake for 18 to 20 minutes. While in the oven, the chocolate cake will rise and puff, doubling in size, but will remain slightly fudgy and dense. At 18 minutes, gently poke the edge of the cake with your finger. The cake should bounce back slightly. Leave the cake in the oven the extra 2 minutes if the cake didn't pass the finger bounce test or if you just aren't sure.
Take the cake out of the oven and let it cool completely on a wire rack before proceeding (in a pinch, you can cool the cake in the fridge or freezer). If storing, wrap the cake tightly in plastic wrap. In the fridge, chocolate cake will keep fresh 5 days. Do not freeze the chocolate cake until it is assembled.
Building the cake: put a piece of parchment or a silpat on the counter. Invert the sheet pan of cake onto it, freeing the cake, and peel off the baked-on parchment or silpat from the bottom.
Use the cake ring to stamp out two circles from the rectangular cake. Reserve them for your top two cake layers. The remaining cake "scrap" will come together to make the bottom layer of the cake.
Layer 1, the bottom: clean the cake ring from stamping and place it in the center of a clean parchment or silpat-lined sheet pan. Take one strip of acetate and line the inside of the cake ring. Reserve the second strip of acetate to use halfway through the cake building process.
Use the back of your hand to tamp the scraps together into a flat, even layer – the base of your cake. Use the back of a spoon to spread half of the apricot jam in an even layer over the cake. sprinkle half of the chocolate crumbs evenly across the top of the apricot jam. Use the back of your hand to anchor them in place. use the back of a spoon to spread half of the Häagen-Dazs® chocolate chocolate chip as evenly as possible over the chocolate crumbs. (Note: temper your pint of gelato in the fridge for 20 minutes for an easy to spread consistency.)
Layer 2, the middle: with your finger, gently tuck the second strip of acetate between the top 1/4" of the first strip of acetate and the cake ring, so that the second strip of acetate forms a clear ring 5"-6" tall enough to support the total height of the finished cake.
Repeat layer 1, using one of the cake rounds as the base. (If one of your two cake rounds doesn't look as good as the other, use it here in the middle and save the pretty one for the top.)
Layer 3, the top: nestle the remaining cake round into the Häagen-Dazs® chocolate chocolate chip of layer 2. Pour the fudge sauce (slightly warm) over the top layer of the cake to coat and cover for a glossy finish.
Transfer the sheet pan holding your assembled cake to the freezer. Freeze the cake for a minimum of 6 hours to set the cake and its fillings. The cake will keep in the freezer for up to 2 weeks.
When you are ready to serve the cake, pull the sheet pan out of the freezer. Using your fingers and thumbs, pop the cake out of the cake ring. Gently peel off the 2 pieces of acetate and transfer the cake to a platter or cake stand and let it defrost in the fridge for a minimum of 3 hours. slice into wedges and serve.
In the fridge, wrap well with plastic. The cake will stay fresh up to 5 days.