Preheat oven to 350F (175C).
Grease and flour (or line with parchment paper) a 13x9 pan. Set aside.
In a large bowl, combine melted butter, sugar, and milk. Stir well.
Add in eggs and vanilla extract and stir until combined.
In a separate bowl, combine flour, baking powder, cream of tartar and salt.
Stir dry ingredients into butter mixture until combined.
Spread batter evenly into prepared 13x9 pan and prepare topping.
In a small bowl, whisk together granulated sugar and cinnamon. Sprinkle evenly over snickerdoodle blondie batter.
Bake on 350F (175C) for 25-28 minutes or toothpick inserted in center comes out clean or with moist crumbs (not wet batter).
