Steep the pistachio cream:
Heat a saucepan over medium heat and add pistachios, stirring 3–5 minutes until toasted. Remove from heat and lightly crush the pistachios into larger chunks but not crumbs. Add 1 cup heavy cream and 1 teaspoon matcha powder, if using, to the saucepan and return to medium heat until the cream just starts to simmer, but not boil. Stir to evenly distribute the matcha. Remove from heat, cover and transfer to the refrigerator to cool completely.
Prepare the crepes:
Place flour, cocoa powder, eggs, milk, ½ teaspoon Kosher salt and melted butter in a blender. Blend until smooth and aerated, about 15 seconds.
Heat a crepe pan over medium heat for 5 minutes. Lightly grease the pan with additional melted butter. Add enough crepe batter to coat the bottom of the pan, about 3 ounces. Use the wooden rateau to evenly spread the batter across the pan.
Cook the crepe until the top has bubbles and is set and the bottom is slightly browned, about 1-2 minutes. Using a wooden spatula, gently release the crepe from the pan and flip. Cook on the other side for 1 minute longer.
Remove the cooked crepe from the pan and place on a piece of parchment. Repeat with the rest of the batter, stacking the crepes with parchment in between each one. You should end up with about 10 crepes.
Whip the pistachio cream:
When cream is completely cooled, add remaining 1 cup heavy cream to the saucepan, then strain out the pistachios. You should end up with about 1½ cups of heavy cream.
Transfer to a stand mixer (or use a hand mixer) and whip the cream until it forms soft peaks. Slowly add powdered sugar, beating again until it forms soft peaks. Add almond and vanilla extracts and mascarpone cheese and beat one final time until the cream is thick and fluffy.
Assemble the crepe roll:
Line a sheet pan with parchment paper, then start by laying two crepes next to each other on the paper so they overlap in the middle. Trim off the top and bottom of each crepe, so they are flat across the top and the bottom. Spread the crepes with a layer of pistachio cream, then carefully coil up from one of the flat ends.
For the next layer, lay one crepe in the middle, then cut a second crepe in half. Lay the two curved halves next to the middle crepe then again, trim off the top and bottom so you have a rectangle. Spread with another layer of pistachio cream, then carefully roll the first crepe coil across it, adding another layer to the coil.
Repeat the process—alternating between two side-by-side crepes and one center crepe to keep the roll even—until you are out of crepes or pistachio cream. You should create about 5 layers in total. Roll up your crepe coil tightly in the parchment paper then transfer to the refrigerator to let it set for at least 1 hour. (You can do this up to 2 days in advance.)
Glaze with the ganache:
About half an hour before you are ready to serve, glaze the crepe roll with the ganache.
Warm the heavy cream in a saucepan until it starts to simmer. Pour the hot cream over the chocolate chips and butter and let it sit for 5 minutes. Mix until smooth, heating the chocolate more over a double boiler, if needed, to get it fully smooth. Stir in cornflakes.
Remove the crepe roll from the refrigerator and unwrap from the parchment paper. Carefully glaze the outside with the crunchy ganache. Transfer back to the refrigerator and let it set for 10-15 minutes.
Slice off the ends of the roll to reveal the coil and transfer to a cake plate or platter and serve.
