Preheat the oven to 350 degrees. Spray an 8 x 8 baking pan with cooking spray and set aside.
In a bowl cream the butter and sugar together. Add in the eggs and vanilla and whisk until blended.
In a small bowl combine the flour, salt, baking powder, and cinnamon.
Stir the dry ingredients into the wet ingredients forming a thick batter. Spread the batter into the bottom of the 8 x 8 pan.
Arrange the apple slices on top of the batter making sure to cover the entire top.
In a small bowl combine the crumb topping ingredients and mix until crumbly. Sprinkle over top of the apples. Bake for 35 minutes.
Remove from oven and let cool for 20 minutes before cutting.