Preheat your oven to 350°F (175°C) and prepare two round cake pans by greasing them with butter or non-stick spray and dusting with flour.

In a large mixing bowl, whisk together all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt.

In a separate bowl, whisk together eggs, vegetable oil, buttermilk, and hot coffee until smooth.

Gently fold the wet mixture into the dry ingredients until just combined.

Divide the batter between the prepared pans and bake for 25-30 minutes or until a toothpick comes out clean.

Melt the semisweet chocolate in a double boiler or microwave and let it cool slightly.

Whip the heavy cream with powdered sugar and vanilla extract until stiff peaks form.

Fold the cooled melted chocolate into the whipped cream.

Assemble the cake by layering one cake layer, spreading half of the mousse, then adding the second layer and the remaining mousse. Refrigerate for at least 3 hours.

For the ganache, heat heavy whipping cream until simmering, then pour it over chopped bittersweet chocolate, corn syrup, and butter, stirring until smooth.

Pour the ganache over the chilled mousse cake and refrigerate until set.