Chocolate Cake:
Preheat the oven to 350°F (175°C). Grease and line two 8-inch round cake pans with tall sides. Set aside.
In the bowl of a stand mixer, whisk together the flour, sugar, cocoa, baking soda, baking powder, and salt until well combined.
In a separate bowl, whisk together the buttermilk, oil, eggs, and vanilla.
With the mixer on low, slowly add the wet ingredients to the dry mixture. Then, carefully mix in the hot coffee. Stir until just combined, scraping the sides of the bowl as needed.
Divide the batter evenly between the prepared cake pans. Bake for 30-40 minutes or until a toothpick inserted comes out clean or with a few moist crumbs.
Let the cakes cool in the pans for 30 minutes, then transfer them to a wire rack to cool completely.
Filling (Ermine Frosting):
In a medium saucepan over medium heat, whisk together the milk and flour. Cook, stirring constantly, until the mixture thickens and resembles a thick pudding, about 3-5 minutes. Remove from heat and allow to cool completely.
In a stand mixer, beat the butter and sugar on medium speed until light and fluffy, about 2-3 minutes.
Add the cooled milk mixture to the butter and sugar and beat on medium-high speed until light and fluffy, about 2-3 minutes.
Switch to the whisk attachment, add vanilla, and continue beating on high for 7-8 minutes until the frosting is smooth and creamy.
Ganache:
Place the chopped chocolate in a heat-safe bowl.
In a microwave-safe bowl, heat the cream for 3-4 minutes, or until it just begins to simmer. Do not allow it to boil.
Pour the hot cream over the chocolate and let it sit for at least 2 minutes. Whisk until smooth and creamy. Set aside to cool slightly before pouring over the cake.
Assembly:
If needed, level the cooled cakes. Place one layer on your cake stand or plate.
Spread the Ermine filling evenly over the cake. Top with the second cake layer.
Pour the chocolate ganache over the entire cake, allowing it to drip down the sides. Let the ganache set for a few minutes before slicing and serving.