How to Prepare Strawberry Mousse Cake
To begin, grease with butter and flour the bottom and sides of a 9-inch springform pan. Reserve the pan for later.
In a large bowl, beat the egg whites together with the cream of tartar until soft peaks form. Gradually add 1/4 cup of the sugar, continuing to beat until stiff peaks form.
In another bowl, beat the egg yolks with the remaining sugar until the mixture is a pale yellow and has a thick consistency.
Pour the egg yolks over the beaten egg whites and add the lemon zest and vanilla, mixing with folding motions carefully so as not to deflate the mixture.
Sift the flour and salt over the mixture and incorporate them gently but thoroughly.
Pour the batter into the prepared pan and bake at 350°F for 25 minutes or until the surface springs back when lightly touched.
Let the cake cool in the pan on a wire rack.
If you wish to prepare it in advance, remove the cake from the pan and wrap it well to store in a cool room temperature for one to two days or in the freezer for up to 2 months.
For the mousse, in a very small saucepan, sprinkle the gelatin over the water and set aside to hydrate.
Rinse, hull, and puree the strawberries until you get a homogeneous mixture.
In a small saucepan, combine the strawberry puree, sugar, and lemon juice and heat gently just until the sugar dissolves. Remove from heat.
Heat the softened gelatin over low heat until it is clear and has a syrup-like consistency. Mix it with the strawberry mixture.
Transfer the preparation to a large bowl and refrigerate until it has the consistency of raw egg white.
Whip the plain yogurt and the whipping cream until stiff peaks form, then carefully fold them into the mousse and refrigerate again.
For the syrup, in a small saucepan, bring the water and sugar to a boil. Remove from heat, let cool, and add the liqueur if desired.
For assembly, using a long serrated knife, cut the cake into two thin layers.
Place the top half, cut side up, on the bottom of a clean 9-inch springform pan.
Pour half of the syrup evenly over the cut side of this layer.
Spread the strawberry mousse over the cake in the pan; it doesn't matter if the mousse overflows from the sides.
Place the remaining layer, cut side down, on top of the mousse, pressing gently to level.
Cover and refrigerate completely, preferably overnight to set.
To finish, release the sides of the pan and, using a wide spatula, transfer the cake to a serving plate. Use a decorative paper if you have planned it in advance.
For the cream that will cover and decorate, rinse and hull more strawberries, then slice them thinly.
Whip the whipping cream until stiff, sweeten it with sugar to taste, and spread evenly over the top and sides of the cake.
Arrange the strawberry slices in a circle around the top and bottom edges of the cake to decorate.
Refrigerate the cake until serving time, which should be within the next one or two hours to enjoy it fresh and delicious.