Preheat the oven to 350°F and line a 9x9" pan with parchment paper.
For the cake, whisk eggs and sugar until tripled in volume. Sift in dry ingredients, fold gently, and transfer to the pan. Bake for ~13-14 minutes. Let the cake cool.
For the ganache, heat cream until just boiling, then pour over chopped chocolate. Cover and wait 5 minutes, then stir until smooth. Cool in the fridge for 20-30 minutes.
For the mousse, whip the chilled cream until stiff peaks form. Gently fold into the cooled ganache in 2-3 additions.
Assembly: Line a 9x9 pan with parchment paper. Place the cooled cake inside and pour over the chocolate mousse. Freeze for 20 minutes.
Decoration: Whip cream and sugar until medium peaks form. When the mousse is somewhat set, top with whipped cream and spread evenly.
Serving: Let the cake cool in the freezer for 1 hour. Dust with cocoa powder and slice into individual servings. Store in the fridge.