Heat oven to 350℉/177℃.
Spray a small sheet pan with cooking spray. Set aside.
In a large bowl, whisk together flour, sugar, baking powder, and salt.
In a medium bowl, whisk together milk, butter, egg, and vanilla.
Pour over flour mixture.
Mix well.
Turn out onto a floured surface.
Roll into a 12-inch x 18-inch rectangle.
Pour melted butter over dough and spread all the way to edges.
Sprinkle with cinnamon sugar.
Beginning with long end, roll dough jellyroll-style.
Using a sharp knife, cut into 1½-inch rolls.
Place on prepared pan.
Bake for 25-30 minutes, or until done.