Preheat oven to 350°F. Grease and line a 9x13-inch pan with parchment paper.
In a large bowl, combine melted butter, 1 1/2 cups granulated sugar, brown sugar, and milk. Stir well.
Add eggs and vanilla extract to the butter mixture, stirring until combined.
In a separate bowl, whisk together flour, baking powder, cream of tartar, and salt. Gradually add the dry ingredients to the wet ingredients, stirring until just combined.
Spread the batter evenly into the prepared pan.
Mix sugar and cinnamon to make a topping. Sprinkle evenly over the batter. Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted in the center comes out clean or with moist crumbs.