Preheat the oven to 350F then line an 8x8 inch baking pan with parchment paper then set aside.
Cream the butter, granulated sugar, and brown sugar until light and fluffy.
1/2 cup butter, 1/3 cup brown sugar, 1/2 cup granulated sugar
Mix in the egg and the vanilla until combined.
1 egg, 1 tsp vanilla extract
Add in the flour, salt, baking powder, cream of tartar, and cinnamon until the flour mixture disappears.
1 & 1/4 cup AP flour, 1/2 tsp baking powder, 1/4 tsp cream of tartar, 1/4 tsp salt, 1 tsp cinnamon
In a small bowl, mix the 3 tbsp granulated sugar and cinnamon together until combined.
3 tbsp granulated sugar, 1/2 tsp cinnamon
Press the cookie dough in an 8x8 inch baking sheet, sprinkle with cinnamon sugar and bake in the preheated oven for 20-22 minutes. Pull out of the oven and cool down before slicing.
Slice into 16 squares and enjoy!
Notes
These bake in a thin, single 8x8 layer, so timing matters more than it would in a 9x13. Pull them the moment the edges look set and the center still looks slightly underdone; they finish cooking in the pan as they cool. A few extra minutes past 22 and they go from chewy to dry.