Separate the egg yolks from egg whites and set whites aside.
Add ½ cup sugar to the egg yolks and beat until creamy, 3-5 minutes.
Add the melted butter, vanilla extract, milk and lemon juice. Mix until combined.
Add the flour and baking powder and mix until combined and set aside.
Beat the egg whites until frothy, then add cream of tartar. Continue whipping while slowly adding the remaining ¼ cup sugar. Beat until still peaks form.
Fold the batter into the egg whites, being careful not to over-mix.
Preheat the oven to 325 degrees.
Line a muffin tin with individual pieces of parchment paper (**See other options below).
Fill each muffin ¾ way full.
Bake for 20 minutes. Turn oven off and crack the door. Leave the sponge cakes in the oven for 20 minutes.
Remove from parchment paper and serve with strawberry sauce (optional).
Strawberry Sauce:
Cut the stems off the strawberries. Combine with mirin in a a blender and puree.
Add puree in a saucepan with the vanilla extract, lemon juice and sugar.
Bring to a boil and simmer for 10 minutes.
Serve with sponge cakes.
Notes
1 - use mini tart dishes instead of a muffin tin. All other instructions remain the same.2 - Use an 8" spring form pan and bake 25-30 minutes.3 - add food coloring or different extracts for different colors and/or flavors.