Combine strawberry milk powder, cornstarch, and egg in a bowl. Whisk until fluffy. Warm whole milk and sugar, then mix into the egg mixture. Strain back into the pot, heat until thickened, and add butter. Cool, cover, and refrigerate.
Making the Dough:
Mix yeast, warm milk, and sugar. Combine with dry ingredients, butter, egg yolks, and vanilla. Knead into a ball, refrigerate overnight.
Frying the Donuts:
Bring dough to room temperature. Roll into a 9x13 inch rectangle, cut out 3-inch rounds. Allow to proof, then fry in oil at 365°F. Toss in strawberry sugar while warm.
Filling the Donuts:
Whip heavy cream to medium-soft peaks. Smooth out pastry cream, fold in the cream, and fill a pastry bag. Poke a hole in each donut, fill with the creamy mixture.
Serve immediately and enjoy your homemade strawberry-filled donuts!