Preheat oven to 375°F. Grease a 9x13" pan with butter.
Whisk together the dry ingredients in a large bowl. Then, stir in the milk, vanilla and melted butter. Pour the batter into the prepared pan and bake for 18 minutes.
While the cake bakes, whisk together the topping ingredients (butter, sugar and cinnamon). Set aside.
After 18 minutes, remove the cake from the oven and whisk the topping again to ensure it's combined. Then, slowly pour the mixture over the top of the cake starting from the edges and working in towards the middle to create an even layer. Gently spread with a spoon if needed.
Place the cake back in the oven and bake an additional 8 - 10 minutes until the topping is bubbly.
Remove from oven and cool completely before slicing and serving.