Whisk the egg yolks and granulated together in a medium bowl until pale and smooth. Whisk in the cornstarch until fully dissolved.

Pour the hot milk into the egg mixture a little at a time while whisking constantly to gently temper the yolks.

Pour the custard mixture back into the saucepan and add the chocolate. Cook over medium-low heat, whisking constantly and scraping the bottom of the pan, until the chocolate completely melts and the custard thickens. It should gently bubble and coat the back of a spoon, about 5-8 minutes.

Remove the pan from the heat and immediately pour the custard through a fine-mesh sieve into a clean bowl to remove any lumps. Let cool to room temperature, then press plastic wrap directly onto the surface and refrigerate until chilled while you prepare the cake layers.

Preheat the oven to 350°F. Line two 8-inch round cake pans with parchment paper and set aside.

In a medium bowl, whisk together the all purpose flour, sugar, cocoa powder, baking powder, baking soda, and salt. Set aside.

In a separate large bowl, whisk the oil, eggs, vanilla extract, and buttermilk until smooth and well combined.

Fold in half of the dry ingredients until mostly incorporated, then slowly stir in half of the hot coffee, scraping the sides and bottom of the bowl. Repeat with the remaining dry ingredients and finish with the coffee. The batter will be thin.

Divide the batter evenly between the prepared pans. Bake for 15-20 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.

Let the cakes cool in the pans for 10 minutes, then turn out onto a wire rack and cool completely before leveling.

Place the egg whites and powdered sugar in the bowl of a 5-quart stand mixer. With the whisk attachment, beat the ingredients on low until combined.

Increase the speed to medium high and beat for 8-10 until a meringue forms. It should be thick and opaque with tips that stand up but bend at the ends. It is ok if your meringue doesn't stiffen all the way as long as it is thick and pure white.

Turn the speed down to low and add the softened butter 1 tablespoon at a time, mixing on medium-low for about 10 minutes.

Level the cake layers with a serrated knife or cake leveler if needed. Place one layer on a serving plate and pipe a tall ring of buttercream around the edge to create a barrier. Fill the center with a generous layer of chilled chocolate custard.

Place the second cake layer on top. With your offset spatula, spread a thin, even layer of frosting over the entire cake to seal in crumbs.

Freeze the cake for about 20 minutes, or until the frosting is dry to the touch. Finish with a thick, smooth coat of frosting.

Decorate with frosting swirls and chocolate jimmies or chocolate shavings. Slice and enjoy!