Prepare the Filling Base

1. In a medium saucepan, whisk together sugar, flour, and salt.
2. Gradually add milk while whisking to ensure no lumps.
3. Cook over medium heat, stirring constantly, until the mixture thickens to a pudding-like consistency.
4. Pour the mixture into a heat-proof container, press wax paper onto the surface, and let cool to room temperature.

Make the Cake

• Preheat oven to 350°F (175°C). Line two 8-inch round cake pans with parchment paper and grease the sides.
• In a large bowl, combine flour, brown sugar, granulated sugar, cocoa powder, baking soda, and salt.
• Add melted butter and oil. Stir until well combined.
• Beat in eggs, egg yolk, and vanilla extract. Mix until smooth.
• Gradually stir in buttermilk, followed by hot coffee. Mix well.
• Divide the batter evenly between the pans and bake for 33–35 minutes, or until a toothpick comes out clean.
• Let cakes cool in pans for 15 minutes, then transfer to a wire rack to cool completely.

Prepare the Filling

1. Beat softened butter in a large bowl until fluffy.
2. Gradually add the cooled filling base, one spoonful at a time, mixing thoroughly after each addition.
3. Stir in vanilla extract and whip until smooth and airy. Chill for 10–20 minutes before using.

Make the Ganache Frosting

1. Combine chocolate and heavy cream in a microwave-safe bowl. Heat for 35 seconds, stir, and repeat in 25-second intervals until smooth.
2. Let the ganache cool to a spreadable consistency.

Assemble the Cake

1. Level the cooled cake layers if necessary.
2. Place the first layer on a serving plate and spread the filling evenly over the top.
3. Add the second layer and smooth the sides.
4. Freeze the cake for 10–15 minutes to firm it up.
5. Spread ganache evenly over the top and sides of the cake. Chill for 15 minutes b