Preheat the oven to 350°F. Grease 2 (8-inch) round cake pans with butter or baking spray. Line the bottom of each pan with a parchment paper circle.

In a large mixing bowl, whisk to combine the flour, sugar, cocoa, baking powder, baking soda, and salt.

In a medium mixing bowl or liquid measuring cup, whisk to combine the buttermilk, vegetable oil, eggs, and vanilla.

Add the buttermilk mixture to the flour mixture and whisk together until combined. Pour in the hot coffee and whisk until well combined.

Divide the batter evenly between the two prepared cake pans.

Bake for 28 to 30 minutes or until a toothpick inserted in the center of each cake layer comes out clean. Let the cake layers cool in the pans for 15 minutes. Then, carefully turn the cake layers onto a wire cooling rack. Remove the parchment paper and let them cool completely, about 1½ hours.

In a medium saucepan, whisk to combine the sugar and the flour. Pour in the milk while whisking constantly until smooth.

Cook the milk mixture over medium heat, whisking frequently, until it starts to simmer, 5 to 7 minutes. Reduce the heat to medium-low and hold it at a simmer, while whisking constantly, until the mixture thickens like pudding, 1 to 2 minutes. Remove it from the heat.

Pour the mixture into a medium bowl and press plastic wrap into the surface so that a skin does not form. Let it cool to room temperature, for about 2 hours. (You can speed this up by putting it in the refrigerator, just make sure that it only cools to room temperature.)

In the bowl of a stand mixer fitted with a paddle attachment, beat the butter on medium speed for 2 minutes until smooth and creamy. Scrape down the sides and bottom of the bowl.

To the bowl with the butter, add the cooled flour mixture 2 to 3 tablespoons at a time, mixing on medium-low speed until fully combined, in between each addition.

Add the vanilla. Increase the mixer to medium speed and beat the mixture for about 1 minute, until the filling is smooth, light and airy. (The frosting should be very thick.)

Place one cake layer top-side down on a cake stand or serving plate. Top with the filling and spread it into an even layer. Top with the second cake layer, top-side down, and press down gently on the top layer to push the filling to the edges of the cake layers. With an offset spatula, smooth the filling around the sides of the cake layers.

Refrigerate the cake while making the ganache, about 20 minutes. (At this point, the cake can be tightly wrapped with plastic and refrigerated for up to 48 hours before coating with the ganache.)

Place the chocolate in a medium, heatproof bowl.

In a microwave-safe, liquid measuring cup, microwave the cream on high heat until steaming, about 90 seconds. Immediately pour the cream over the chocolate. Let it stand for 5 minutes.

Stir until the chocolate is melted and the mixture is smooth and thickened. Let it cool until it’s thick enough to spread into a thin layer all over the cake (without sliding off of the cake or running down the sides too quickly), about 15 minutes.

Dollop the slightly cooled ganache all over the top of the cake. With a small, offset spatula, spread it into a thin layer on top of the cake and gently push the ganache over the edges of the top layer to run down the sides. Spread the ganache into an even layer all over the cake.

Let the ganache set completely before slicing. Store the cake covered in the refrigerator for up to 4 days.