Preheat oven to 350°F and line an 8 x 8 square pan with parchment paper and set aside.
In a large bowl combine melted butter and sugars and stir until well combined. Add vanilla and combine. Add eggs one at a time and stir until fully incorporated. Add flour, cornstarch, cream of tartar and stir until just combined.
Pour batter into prepared pan and spread so it is distributed evenly.
In a small bowl combine 1/4 cup sugar and cinnamon and sprinkle over the top of the batter.
Bake in preheated oven for 35 minutes or until the bars just start to pull away from the edge and a toothpick comes out clean.