Preheat oven to 350 degrees F. Grease a 12 x 9-inch baking pan and set aside.

Melt the butter in the microwave and combine with the sugar. Add in eggs, one at a time, mixing well after each one. Add vanilla.

In a mixing bowl, combine together flour, baking powder, and cinnamon. Gradually add the dry ingredients to the wet ingredients, and mix until combined and smooth. The batter will be very thick, but that's ok.

Pour half of the batter into the prepared baking pan. Using a palette knife, spread to make an even surface and place apples in a single layer over the top. Gently press them into the batter. Spread the remaining batter over the top and carefully lever the surface. Scatter the rest of the apple slices over the batter and press them lightly into the surface.

Bake for about 45–55 minutes or until toothpick inserted in the center comes out clean. Let cool and sprinkle with powdered sugar.