Preheat oven to 350 degrees F. Grease an 8x8'' baking dish.
In a large bowl combine the flour, sugar, baking powder, salt, milk, egg and vanilla. Stir in melted butter. Pour mixture into prepared pan and smooth into an even layer. (This will be your base)
Make the cinnamon sugar filling by combing all ingredients together until smooth.
Drop small spoonfuls of the mixture evenly over the top of the cake batter. Use a knife to smooth over the cake.
Bake 30-37 minutes or until a toothpick inserted in the center comes out clean. Make your frosting and spread over your cake once it is cooled.
The cake is best served the same day.