Heat the oven to 175 C / 347 F. Grease a 23 cm / 9-inch springform pan and line the bottom with parchment.

Cream the butter and sugar until pale, then beat in the egg yolk. Add the flour, baking powder, and salt and mix to a soft dough. Press it evenly into the bottom of the pan.

Whisk the egg whites to soft peaks, then gradually add the sugar and whip to stiff, glossy peaks. Fold in the ground almonds gently with a spatula.

Spread the meringue evenly over the dough base and scatter the flaked almonds on top.

Bake for 35 to 40 minutes, until the meringue is golden brown and feels set, not wet, when pressed lightly.

Cool the cake completely in the pan on a wire rack, about 90 minutes, before releasing the springform sides.

Heat the milk in a 2-quart / 2 L saucepan over medium heat until steaming, not boiling.

Whisk the egg yolks with the sugar and cornstarch in a bowl until pale. Slowly pour in the hot milk, whisking constantly.

Return the mixture to the saucepan and cook over medium-low heat, stirring constantly, for 3 to 4 minutes until it thickens enough to coat the back of a spoon. Do not let it boil hard or the yolks will scramble.

Strain the custard through a fine-mesh sieve into a bowl, stir in the vanilla, and cover with plastic wrap touching the surface. Cool to room temperature, about 30 minutes.

Beat the softened butter in a mixer until fluffy. Add the cooled custard a spoonful at a time, beating until smooth and creamy.

Spread the custard cream over the cooled cake. Chill for at least 2 hours before slicing with a knife wiped clean between cuts.