Preheat the oven to 375 degrees F (190 degrees C). Grease a 7-inch square baking tin, or line with paper liners.

Whisk brown sugar, applesauce, oil, eggs, and vanilla extract together in a large bowl.

Combine flour, cinnamon, and baking powder in a separate bowl. Stir into egg mixture until moistened.

Spoon 1/2 the batter into the prepared baking tin and spread evenly. Place apple slices on top and cover with remaining batter.

Bake in the preheated oven until the top springs back lightly when pressed, about 30 minutes. Remove from the oven and let cool completely before removing from the tin and cutting into bars, about 30 minutes.