Mix 1 cup of sugar, 5 tablespoons of flour, and ¼ teaspoon of salt in a medium saucepan. Slowly pour in 1 cup of whole milk while still whisking until combined.

Stir the mixture on the stovetop on medium heat until it thickens like pudding.

Turn off the stovetop and pour the mixture into a heatproof bowl. Press a piece of wax paper (or a towel) down to the surface of the filling base. Put aside.

Preheat your oven to 350°F or 175°C. Line two 8-inch round pans with parchment paper, grease them with oil and sprinkle some flour on the sides of the pans. Set them aside.

Whisk 1 ¾ cups of flour, 2 cups of brown and granulated sugar (1 cup each), ¾ cup of cocoa powder, ¾ teaspoon of salt, and 1 ½ teaspoons of baking powder in a large bowl.

Dump in ½ cup of vegetable oil and ½ cup of melted butter. Beat them well using an electric stand or hand mixer with a paddle attachment until incorporated.

Fold in 2 large eggs, 1 egg yolk, and 2 teaspoons of vanilla extract until combined. Gradually pour in 1 cup of buttermilk and mix it until blended. Continue with the hot coffee and stir until well combined.

Pour the batter into two greased pans and bake at 350°F for 33 to 35 minutes. Let them rest in the pans for around 10 to 15 minutes. Then, free them out of the pans and allow them to cool completely on a wire rack.

Make the chocolate ganache

Microwave 1 cup of heavy whipping and 8 oz of semi-sweet chocolate on high for about 35 seconds. Whisk the mixture well and microwave for another 25 seconds. Stir until spreadable; if not, put it in the microwave for another 25 seconds. Set it aside to cool.

In a mixing bowl, beat 1 cup of butter on high speed using a stand or hand mixer with a paddle or whisk attachment until fluffy and light.

Switch the mixer to medium speed and fold in 1 spoonful of the base mixture at a time. Repeat until you finish with the filling base.

Toss in a teaspoon of vanilla extract, switch to medium-high speed, and whip the creamy filling until airy and smooth. Smooth out the air pockets using a spatula. Place the frosting bowl in the fridge for 10 to 20 minutes to set.

Put the first cake layer on your serving plate and top it with an even layer of filling. Place the remaining cake layer on the frosting. Smooth out the side with a spatula.

Let the cake chill out in the freezer for about 15 minutes before dressing it with the chocolate ganache. If the ganache firms up, microwave it for 5 to 10 seconds until spreadable but not liquidy. Then, let it cool again.

Spread chocolate ganache on top and around the cake. Put the entire cake into the fridge for 15 minutes. Then, slice it and enjoy!