- Preheat the oven to 180 °C (350 °F).
- Line a 38 x 25-cm (15 x 10-inch) Pyrex pan with parchment paper. Lightly butter the paper.
- In a bowl, combine the flour, cocoa powder, salt and baking powder.
- In another bowl, whisk the eggs with the sugar with an electric mixer for 5 minutes.
- Gently fold in the dry ingredients alternating with the butter. Spoon the batter into the prepared pan. Bake in the centre of the oven for about 13 minutes.
- Right out of the oven, flip the cake onto a parchment paper dusted with sugar. Remove the baking paper. Let cool.
- In a small bowl, sprinkle the gelatin over the water. Allow to bloom for 2 minutes.
- In a blender, purée the strawberries. Pour into a saucepan and gently heat without boiling. Dissolve the gelatin in the microwave and add to the puréed strawberries. Cool to room temperature.
- In a bowl, beat the egg whites until soft peaks form. Add the sugar and beat until stiff peaks form. Stir in the strawberry purée and refrigerate while assembling the cake.
- Line a 23 x 13-cm (9 x 5-inch) loaf pan with parchment paper letting the paper hang over 2 opposites sides. In the sheet cake, cut a 20 x 10-cm (8 x 4-inch) rectangle and place at bottom of the pan. Cut two 23 x 5-cm (9 x 2-inch) pieces and cover the walls on both sides. Spread the mousse in the pan. Cover with a 23 x 13-cm (9 x 5-inch) piece of sponge cake. Cover with plastic wrap and refrigerate for 4 hours.
- Slice the cake and serve cold. Garnish with strawberries. This cake must be eaten within 48 hours.