Making this strawberry cream cake requires careful attention to technique, starting with the delicate sponge cake base. Begin by preheating your oven to 375°F and preparing two 9-inch round cake pans with butter and flour.
In a small bowl, whisk together 1/2 cup flour, 1/3 cup cornstarch, and 1 teaspoon baking powder. Separate your 6 eggs, beating the yolks in a small bowl until frothy. In a large bowl, beat the egg whites with a pinch of cream of tartar until stiff peaks form, then gently fold in the beaten yolks and dry ingredients alternately to maintain the airiness.
Divide the batter between prepared pans and bake for 30 minutes, or until the center springs back when lightly touched.
While the cakes cool, prepare the custard filling by combining 1/4 cup sugar and 1 tablespoon cornstarch in a saucepan. Gradually whisk in 1 1/2 cups half-and-half, followed by 3 egg yolks and 2 tablespoons butter.
Cook over medium-low heat, stirring constantly, for about 10 minutes until the mixture thickens to a custard consistency. Remove from heat, cool to room temperature, then stir in 1 tablespoon vanilla. Reserve several whole strawberries for garnish and slice the remainder.
Assembly creates the cake's impressive four-layer structure by cutting each cooled cake layer in half horizontally. Place the first layer cut-side up on your serving plate, spread with half the custard, then add the second layer cut-side down.
Spread 1/2 cup strawberry jam over this layer and arrange the sliced strawberries on top. Add the third layer cut-side up, spread with remaining custard, then top with the final layer cut-side down. The cake can be refrigerated at this point for up to 4 hours.
One hour before serving, whip 1 1/2 cups chilled cream to soft peaks, then fold in 2 tablespoons powdered sugar and 1 teaspoon vanilla. Cover the entire cake with this whipped cream and garnish with the reserved whole strawberries.