With the rack in the middle position, preheat oven to 350° F (180° C). Grease an 8-inch (20-cm) square baking pan. Set aside.
In a bowl, combine the flour, baking powder, and salt. Set aside.
In another bowl, beat the egg whites and cream of tartar with an electric mixer until soft peaks form. Gradually add 100 grams (1/2 cup) of the sugar, beating until stiff peaks form. Set aside.
In a third bowl, beat the remaining sugar, the butter, egg yolks, and vanilla with an electric mixer. At low speed, add the dry ingredients alternately with the milk.
Stir a quarter of the egg whites into the batter to lighten it. With a spatula, gently fold in the remaining egg whites. Pour into the prepared pan.
Bake for about 35 minutes or until a toothpick inserted in the center of the cake comes out clean. Unmould immediately and let completely cool on a wire rack.
To serve, cut into 9 squares and then slice each in half horizontally to make two thin squares. Create a stack of cake, strawberries, whipped cream, cake, strawberries, whipped cream.