Heat the oven to 375 degrees.
Grease a 9-inch cast iron skillet or an oven safe nonstick pan with cooking spray. Be sure to thoroughly cover the inside edges to ensure the pancake doesn't stick and for ease of flipping the pancake after it's baked. Add the butter to the skillet on medium- low heat. Once the butter is melted, pour the maple syrup into the skillet.
Place the apple slices on the bottom of the skillet and simmer for 2-3 minutes on each side. Sprinkle the cinnamon over the apples and turn off the heat.
Combine the flour, sugar, baking powder, baking soda and salt in a large bowl. Set aside.
Melt the remaining 2 tablespoons of butter in the microwave for about 30 seconds or until melted.
Combine the milk and egg in a bowl and mix throughly. Add the melted butter slowly while whisking the mixture.
Add the wet ingredients to the dry ingredients and mix well with a whisk.
Pour the batter in the skillet. Pour it evenly over the apple slices.
Place the skillet in the oven and bake for 20-25 minutes or until a toothpick or knife can be inserted into the middle and comes out clean (without any wet batter stuck to it).
Carefully flip the pancake onto a plate. Slice and serve on its own or with additional maple syrup.