Preheat your oven to 325°F (165°C) and line a 10x15 inch baking sheet or jelly roll pan with parchment paper.

In a large bowl, cream together the softened butter and 2/3 cup sugar until light and fluffy, then beat in the egg yolks one at a time along with the vanilla extract.

Sift the flour and baking powder together, then gently fold them into the butter mixture alternately with the milk until a thick, smooth batter forms.

Spread the cake batter evenly into the prepared baking pan using an offset spatula; the layer will be quite thin.

In a perfectly clean bowl, whip the egg whites until soft peaks form, then gradually add 1 cup of sugar while beating until stiff, glossy peaks form.

Spread the meringue evenly over the raw cake batter layer in the pan and sprinkle the top generously with sliced almonds.

Bake in the preheated oven for 30 to 35 minutes, or until the meringue is dry and golden brown, then let it cool completely in the pan.

While the cake cools, whip the heavy cream until soft peaks form and gently fold it into the chilled vanilla custard to create the filling.

Once cool, lift the cake out of the pan and cut it vertically into two equal halves.

Place one half of the cake onto a serving platter, meringue side up, and spread the vanilla cream filling evenly over it.

Carefully place the second half of the cake on top of the filling, meringue side up, and chill for at least 1 hour before slicing to allow the flavors to meld.