- With the rack in the middle position, preheat the oven to 180 °C (350 °F). Line a 43 x 31-cm (17 x 12-inch) baking sheet with parchment paper. Generously butter the sides of the baking sheet.
- In a bowl, beat the eggs, sugar, and vanilla with electric mixer until light and fluffy, about 5 minutes.
- In another bowl, sift the flour with the baking powder. Fold into the previous mixture using a spatula. Drizzle in the butter, folding gently. Pour half of the batter at one end of the baking sheet.
- Stir the cocoa powder into the remaining batter and pour it at the other end of the baking sheet.
- With a spatula, spread the white and the chocolate batters until they meet in the centre of the baking sheet.
- Bake for about 13 minutes or until a toothpick inserted in the middle of the cake comes out clean.
- Right out of the oven, invert the cake onto parchment paper sprinkled with sugar. Remove the baking paper and let cool to room temperature.
- In a saucepan, gently melt the chocolate. Remove from the heat and whisk in the butter and egg yolks.
- In a bowl, whip the cream with the sugar.
- In another bowl, beat the egg whites with the cream of tartar until stiff peaks form. Fold about 125 ml (1/2 cup) of the egg whites into the melted chocolate mixture and then add the remaining meringue, folding gently with a spatula. Add and fold in the whipped cream.
- Slice the cake across its width into 2.5-cm (1-inch) by 31-cm (12-inch) strips.
- In a 1.5 litres (6 cups) bowl, place the strips alternating vanilla and chocolate cake, starting at the centre. Firmly pack the pieces to fit the shape of the bowl. With a bread knife, cut the end of each strip so that they finish at the edge of the bowl.
- Fill with the mousse and top with the remaining cake strips. Cover with plastic wrap and refrigerate for 4 hours.
- To unmould, place a plate on the bowl and invert. Slice and serve.