Preheat oven to 350 degrees and grease a 8x8 inch baking dish.
First make cake batter. Cream together the butter and sugar.
Add egg and vanilla extract to butter and sugar and mix. Blend until combined.
In a separate bowl mix together flour, baking powder and salt. Add half of the dry ingredients to the wet ingredients and mix. Once mostly combined add milk and then the rest of the dry ingredients.
Now make the filling! In a separate small bowl pinch together all the filling ingredients until it feels like soft wet crumbs. (It will almost feel like wet sand)
Lastly the topping, almost done! Pinch together all topping ingredients until you achieve a similar texture as filling. (Texture will slightly vary because the ingredients differ slightly)
Fill 8x8 greased baking dish with half of cake batter. Pour filling over cake batter then add remaining cake batter on top. (Try to gently spread out second layer of cake batter, this will be a little tough because of the filling)
Top off the cake with the topping and bake in oven for 45-50 minutes. You will know it is done when a toothpick comes out clean when testing the cake.