Preheat your oven to 350°F (175°C) and prepare a 9×13-inch baking dish by greasing it and lining with parchment paper.

In a large mixing bowl, whisk together melted butter, brown sugar, and granulated sugar until smooth and creamy. Add eggs and vanilla extract, whisking until well incorporated.

In a separate bowl, sift together flour, cream of tartar, baking soda, and salt. Gradually add to the wet mixture, stirring gently until just combined.

In a small bowl, mix together granulated sugar and cinnamon with melted butter until sandy in texture. Set aside.

Spread half of the blondie batter into the prepared baking dish. Sprinkle cinnamon-sugar mixture over the batter and dollop remaining blondie batter on top, swirling carefully.

Bake for 30 to 35 minutes until edges are golden brown and a toothpick inserted comes out with moist crumbs. Cool for 20 minutes before slicing.