Flourless Chocolate Cake with Whipped Ganache

Preheat oven to 325°F (160°C). Sift together the cocoa powder, cornstarch, and salt; set aside. In a saucepan or microwave-safe bowl, melt the butter and add 5 oz (145g) of chopped 70% dark chocolate. Stir until the chocolate has fully melted and the mixture is smooth and glossy. Add the sugar and mix until well combined. Add the 3 cold eggs one at a time, fully incorporating each egg before adding the next. Gradually add the dry ingredients (sifted cocoa, cornstarch, salt) and stir to combine until just mixed. Pour the batter into a 9x9 inch baking pan and bake for 15-18 minutes. Once the cake has fully baked, remove it from the oven and allow it to cool completely before assembling with the whipped ganache. 

For the whipped ganache: Gently heat 1.75 cups of heavy cream until steaming (do not boil). Pour the hot cream over the 6 oz of chopped semisweet chocolate. Whisk until smooth and place the ganache mixture in the fridge to cool slightly. Every 10 minutes or so, give it a good whisk, repeating this process until the ganache is cool and thickened to a consistency suitable for whipping. Whip the remaining heavy cream to medium-soft peaks. Using a rubber spatula, gently fold the prepared (cooled and slightly thickened) chocolate ganache into the whipped cream. Return the cooled cake to a parchment-lined cake pan and cover it evenly with the whipped ganache. Use an offset spatula to level the ganache, then place the cake in the fridge to cool completely and set. Dust with unsweetened cocoa powder before serving.