Preheat oven to 350°F. Grease a 10" square pan (8" or 9" works as well). Whisk together the flour, sugar, baking powder, and salt. Stir in the milk, vanilla, and butter. Pour the batter into the pan and bake for 25 minutes (add 2 minutes for an 8" pan).
While the cake is baking, combine the topping ingredients and whisk together. Remove the cake from the oven when the timer goes off and pour the topping across the cake.
Bake the cake for an additional 10 minutes until the cinnamon layer is bubbling. Let cool before serving.