1. Preheat the oven to 350°F. Grease and line an 8-inch square baking pan with parchment paper.

2. In a medium bowl, whisk together the flour and granulated sugar. In a separate bowl, beat the eggs until light and fluffy.

3. Gradually fold the egg mixture into the flour mixture until just combined. Pour the batter into the prepared pan and bake for 15-18 minutes, or until a toothpick inserted in the center comes out clean.

4. Allow the cake to cool completely in the pan, then use a serrated knife to slice the cake horizontally into 3 equal layers.

5. In a medium saucepan, heat the heavy cream over medium heat until just simmering. Remove from heat and stir in the chopped dark chocolate until smooth and fully melted. Allow the ganache to cool slightly.

6. In a medium bowl, beat the softened butter until light and fluffy. Gradually add the brewed coffee and beat until well combined.

7. Place one layer of the sponge cake on a serving platter. Spread half of the coffee buttercream evenly over the top. Top with another layer of sponge cake and the remaining buttercream. Top with the final layer of sponge cake.

8. Pour the slightly cooled chocolate ganache over the top of the cake, allowing it to drip down the sides. Refrigerate the cake for at least 2 hours before serving.

9. Just before serving, dust the top of the cake with powdered sugar.