How to Make It
Make the Cake: Preheat oven to 350°F. Line the bottom of a 9-inch springform pan with parchment paper and grease with cooking spray. Set aside. In a bowl, whisk together the flour, baking soda, salt, and cocoa powder until well combined. In a separate large bowl, whisk together brown sugar, coffee, oil, and vinegar until sugar is dissolved. Add dry ingredients to wet ingredients and stir until completely incorporated. Pour into prepared springform pan and bake until a toothpick inserted comes out clean, about 35 minutes. Set aside to cool.
Make the Mousse: Heat 1 cup cream over medium-high until just beginning to steam. Remove from heat. Whisk together egg yolks and sugar in a bowl and slowly stream in half of the hot cream while whisking. Add egg yolk mixture into saucepan with remaining hot cream and whisk to combine. Return to medium-high heat, whisking constantly until slightly thickened and mixture begins to boil. Remove from heat and whisk in melted chocolate until completely smooth. Set aside to cool to room temperature, about 1 hour and 15 minutes. Whip the remaining 1 cup cream with the powdered sugar. In 3 additions, fold the cream into the chocolate with a spatula. Pour over top of cake layer in springform pan and smooth the surface with a spatula. Refrigerate for 5 hours or overnight, until the mousse is completely set.
Make Ganache: Heat cream in a saucepan until just beginning to steam. Remove from heat and add chocolate. Let sit for 2 minutes before stirring to fully melt the chocolate into the cream. Allow to cool to room temperature. (Note: Do not refrigerate.)
Assemble the Cake: Take a hot wet towel and run it around the outside of the springform pan a few times to loosen the cake. Open the pan, gently remove the base and parchment paper, and move the cake to a platter. Spread the cooled chocolate ganache over the top of the cake and garnish with fresh berries and whipped cream if desired.