Heat oven to 350 degrees. Butter a 9-inch round cake pan. Line the bottom of the baking pan with a round of parchment paper.

Melt butter in a large saucepan. Once melted, remove from heat and stir in chocolate until melted. Stir in 2 tablespoons water, then baking powder, flour and cocoa until just combined.

In a medium bowl, whisk egg yolks with sugar, 1 tablespoon water and vanilla until pale and no longer lumpy. Stir into chocolate mixture until just combined.

In a larger, very clean bowl, beat egg whites and salt until stiff. Gently fold into chocolate mixture until most of the white streaks disappear.

Bake for 25 to 30 minutes, until a toothpick inserted into the center of the cake comes out batter-free. Remove from oven and rest on rack for 5 minutes. Run knife along outside cake to make sure it isn’t sticking anywhere, then invert onto another rack, then back onto a cake plate. Eat warm or cold.

To make a layer cake, make chocolate whipped cream several hours before filling the cake. Heat 1 cup heavy cream to a simmer, remove from heat, whisk in 6 ounces chopped semisweet chocolate until fully melted. Stir in a second cup of cold heavy cream. Chill in the fridge for at least four hours.

Make twice the cake recipe, divide into 3 9-inch cake pans, and bake for approximately 15 to 20 minutes. Let cool completely.

Whip fully chilled chocolate cream into whipped cream. Spread half of the chocolate whipped cream on the first cake layer, then top with the second. Spread second half of chocolate whipped cream on second cake layer, then top with the third. Keep cake in fridge until needed.