Preheat your oven to 350°F (175°C). Grease and line an 8x8-inch or 9x9-inch baking pan with parchment paper.
In a large bowl, whisk together the melted butter, brown sugar, and granulated sugar until smooth. Add the eggs and vanilla extract, and mix well. Stir in the flour, cream of tartar, baking soda, salt, and cinnamon until just combined. Do not overmix.
Spread the batter evenly in the prepared pan. In a small bowl, combine the sugar and cinnamon for the topping and sprinkle it evenly over the batter. Bake for 25–30 minutes, or until the center is set and a toothpick inserted in the middle comes out clean or with moist crumbs.
Let the blondies cool completely in the pan before slicing into squares. This helps them set and enhances the chewy texture.