Russian Apple Upside-Down Cake

Combine egg whites with sugar in a large mixing bowl and whisk vigorously using an electric mixer until the mixture reaches a light, airy consistency. Gradually incorporate egg yolks and vanilla extract into the mixture using the same mixer.

In a separate, clean bowl, gently combine flour and baking powder until well-blended. Gradually merge the dry ingredients into the egg mixture, mixing until just combined.

In a 9x13-inch baking pan, layer butter and sliced apples at the bottom. Place the pan in the oven to preheat it to 350 degrees F (175 degrees C). As the butter melts, stir in the apples and melted butter until well-coated.

Pour the cake batter over the apple-butter mixture in the pan. Bake for 20 minutes, or until the top is golden brown.

Remove the cake from the oven and loosen its edges using a spatula. Carefully flip the cake over onto a serving platter to display. Allow it to cool for a short time before serving, or serve immediately.