In a medium saucepan, whisk together granulated sugar and egg yolks until light and creamy. Gradually mix in cornstarch, all-purpose flour, and vanilla extract, ensuring there are no lumps.

In another saucepan, heat whole milk over medium heat until it simmers. Slowly whisk the hot milk into the egg mixture, then return it to the heat and cook until thickened, about 2-3 minutes. Remove from heat and stir in unsalted butter until melted.

Transfer the pastry cream to a bowl and cover with plastic wrap, pressing the wrap directly onto the surface to prevent a skin. Cool to room temperature.

In a separate bowl, whip heavy cream on medium speed until soft peaks form. Gently fold it into the cooled pastry cream until fully incorporated. Refrigerate for an hour.

Beat heavy cream, mascarpone, and powdered sugar together until fluffy with stiff peaks. Set aside.

Cut the angel food cake into 1-inch cubes.

In a 9x13-inch pan, layer half of the cake cubes at the bottom, spread blueberry pie filling, add another layer of cake cubes, and then spread diplomat cream and chantilly cream over the top.

Cover the assembled cake with plastic wrap and refrigerate for at least 4 hours, preferably overnight.

Once chilled, remove from refrigerator, top with toasted almond slivers, slice, and serve with fresh blueberries or whipped cream.