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.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.