Preheat your oven to 325 degrees.
Grease a 9X13 inch baking pan. Set aside.
In a large mixing bowl combine the sliced apples with the cinnamon and sugar.. Toss until all the apple slices are evenly coated. Set aside.
In a separate mixing bowl whisk together the flour, baking powder and baking soda. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment beat the butter and sugar together on medium speed until fluffy, about 5 minutes. Add in the eggs and vanilla and mix until combined. Add in the dry ingredients little by little alternating with the buttermilk, making sure to end with the dry ingredients. Mix until just combined. Do not over mix.
Spread half the batter over the bottom of the prepared pan. Spread the apples over the cake batter in an even layer. Drop the remaining cake batter over the apple layer in spoonfuls. Don't worry about making it perfect.
Stir together the melted butter, brown sugar and cinnamon in a small bowl and pour over the top of the cake. Bake for 50-55 minutes or until an inserted toothpick comes out clean. Cool before slicing.