Preheat the oven to 350 degrees F. Line a 9×13 baking pan with parchment paper leaving overhang to use as handles for easy removal. Grease the exposed areas of the baking pan.
In a medium bowl whisk together the flour, cream of tartar, baking powder, and kosher salt. Set aside.
2 cups All-purpose flour, 1/2 tsp Cream of tartar, 1 tsp Baking powder, 1/2 tsp Kosher salt
In a large bowl, mix the melted butter and sugars until combined. Add the eggs and vanilla extract and mix until fully incorporated.
3/4 cup Unsalted butter, 1 cup Granulated sugar, 3/4 cup Dark brown sugar, 2 large Eggs, 1 1/2 tsp Vanilla extract
Add the dry ingredients to the wet ingredients and mix until just combined. Do not overmix.
In a small bowl mix together the granulated sugar and ground cinnamon.
1/4 cup Granulated sugar, 1 1/2 tsp Ground cinnamon
Spread the batter evenly into the prepared pan. Using a spoon sprinkle the cinnamon topping evenly over the top.
Bake for 25-30 minutes, until it’s golden brown and a toothpick inserted into the center comes out clean.
Allow the bars to cool completely on a wire rack, remove them using the overhang handles, and slice into desired size servings.