In a separate bowl, beat the yolks with the remaining 1/4 cup sugar until light yellow and thickened. Fold the yolks into the whites along with the grated rind of 1/4 teaspoon lemon and 1 teaspoon vanilla. Gently fold in 1/2 cup all-purpose flour mixed with 1 pinch salt. Pour the batter into the prepared pan and bake at 350°F for 25 minutes until the top springs back when lightly touched. Allow the cake to cool completely in the pan on a wire rack.

While the cake cools, prepare the strawberry mousse filling. Sprinkle 1.5 envelopes of unflavored gelatin over 1/4 cup cold water in a small saucepan and let it soften. Rinse, hull, and puree 2 cups strawberries, then combine the puree with 1/3 cup granulated sugar and 2 tablespoons lemon juice in a saucepan.

Heat gently just long enough to dissolve the sugar, then remove from heat. Warm the softened gelatin over low heat until clear and syrupy, then stir it into the strawberry mixture. Transfer to a large bowl and chill until the consistency resembles raw egg white, then whisk in 1/2 cup plain yogurt. Whip 1/2 cup whipping cream until it forms soft peaks and fold it into the mousse, then refrigerate.

To assemble, use a long serrated knife to cut the cooled cake into two thin layers. Prepare a simple syrup by bringing 3 tablespoons water and 3 tablespoons granulated sugar to a boil, cooling it, and stirring in 2 tablespoons strawberry jam or kirsch liqueur.

Place the top cake layer cut-side up in a clean 9-inch springform pan, drizzle half the syrup over it, then spoon the mousse over the cake. Top with the remaining cake layer cut-side down, pressing gently, then cover and chill overnight. Before serving, release the pan and transfer the cake to a serving plate.

Whip 1 1/2 cups whipping cream with 2 tablespoons granulated sugar until stiff, then spread it over the top and sides. Arrange thinly sliced fresh strawberries from 2 cups around the top and bottom edges and refrigerate until serving, within one to two hours.