Preheat oven to 350°F. Grease a large baking sheet pan (18"x13") with butter. Whisk together the dry ingredients. Stir in the milk, vanilla and butter. Pour the batter into the pan and bake for 23 minutes.
While the bars are baking, combine the topping ingredients and whisk together. Remove the bars from the oven after 23 minutes and pour the topping over them. Bake an additional 8 minutes, until the cinnamon layer is bubbling. Let cool before slicing into bars and serving. Enjoy!
Notes
Pour the topping around the sides of the bars first and then lightly across the top. I've found that if I pour it all on the center of the bars immediately, that will cause them to fall slightly. Still delicious that way, but not as pretty.