Syrup:

Sponge cake: Using a stand mixer, beat the egg whites until foamy, then add half the sugar and beat until stiff peaks form. In a separate bowl, whisk the yolks with the remaining sugar. Add the melted butter, baking powder, and flour. Gently fold the beaten egg whites into the batter with a spatula. Once the batter is smooth, pour it onto a baking sheet lined with parchment paper (large enough to cut out two cake rounds). Bake for 10 minutes at 375°F (190°C).

When the cake comes out of the oven, brush it with syrup and cut out your cake rounds.

Diplomat cream: Soak the gelatin sheets in a bowl of cold water for about 10 minutes. In a bowl, whisk the egg yolks with the sugar until pale. Add the cornstarch. In a saucepan, heat the milk, butter, and vanilla bean. Add the squeezed gelatin sheets. Pour 1/3 of the hot milk over the egg mixture, whisking quickly. Then add the rest of the milk and return everything to the saucepan over medium heat, whisking constantly. The cream will thicken.

Transfer the cream to a bowl. Cover with plastic wrap directly on the surface and let cool. Using a stand mixer, whip the heavy cream until stiff peaks form. When the pastry cream is just warm (not too hot or the whipped cream will melt, not too cold or the cream will set from the gelatin), gently fold it into the whipped cream with a spatula. Transfer the diplomat cream to a piping bag and chill for 10 minutes.

Assembly: Line your cake ring with a strip of acetate (Rhodoïd) around the inside. Place the first cake round in the bottom. Arrange halved strawberries around the edge, cut side facing out. Spread a layer of diplomat cream, then add chopped strawberries. Place the second cake round on top. Spread the remaining cream to the top, smooth it out, and refrigerate overnight.