Prepare the Chocolate Cake Layer: Preheat oven to 350°F (175°C). Grease and flour one 9-inch round cake pan, line bottom with parchment. Sift together flour, sugar, cocoa powder, baking soda, baking powder, and salt. In another bowl, whisk together eggs, buttermilk, hot coffee, oil, and vanilla. Pour wet ingredients into dry and mix for 2 minutes until smooth. Pour into prepared pan and bake for 28-32 minutes until toothpick comes out clean. Cool for 10 minutes, then turn out onto wire rack to cool completely.

Prepare the Vanilla Cake Layer: Preheat oven to 350°F (175°C). Grease and flour one 9-inch round cake pan, line bottom with parchment. Whisk together flour, baking powder, and salt. Set aside. Beat butter and sugar until light and fluffy, about 4 minutes. Add eggs one at a time, then vanilla extract. Add flour mixture in three additions, alternating with milk, beginning and ending with flour. Pour into prepared pan and bake for 25-28 minutes until golden and toothpick comes out clean. Cool for 10 minutes, then turn out onto wire rack to cool completely.

Make the Vanilla Pastry Cream: In a saucepan, whisk together sugar, cornstarch, and salt. Whisk in egg yolks until smooth and pale. Gradually whisk in milk. Cook over medium heat, whisking constantly, until mixture thickens and boils, about 8-10 minutes. Continue cooking for 1-2 minutes more. Remove from heat, stir in butter, vanilla, and almond extract if using. Transfer to bowl, cover surface with plastic wrap, and refrigerate until completely chilled, at least 3 hours.

Make the Chocolate Pastry Cream: In a saucepan, whisk together sugar, cornstarch, cocoa powder, and salt. Whisk in egg yolks until smooth. Gradually whisk in milk. Cook over medium heat, whisking constantly, until mixture thickens and boils. Remove from heat and stir in chopped chocolate, butter, and vanilla until smooth. Transfer to bowl, cover surface with plastic wrap, and refrigerate until chilled, at least 3 hours.

Make the Chocolate Ganache: Place chopped chocolate in a heatproof bowl. Heat heavy cream until just simmering. Pour over chocolate and let sit for 2 minutes. Add butter and corn syrup, then whisk until smooth and glossy. Let cool for 30-35 minutes until spreadable.

Assemble the Cake: Level both cake layers by trimming domed tops. Place chocolate cake layer on a serving plate. Spread all of the vanilla pastry cream evenly over the chocolate layer. Place vanilla cake layer on top. Spread all of the chocolate pastry cream evenly over the vanilla layer. Refrigerate for 30 minutes to set the layers. Pour chocolate ganache over the top, spreading to the edges and down the sides. Use an offset spatula to create smooth, textured patterns on the sides. Create decorative swirls on top with remaining ganache. Refrigerate for at least 2 hours before serving.

Serve: Remove from refrigerator 20 minutes before serving. Slice with a hot, sharp knife, wiping clean between cuts.