Preheat oven to 350°F (175°C). Grease and line an 8x8 inch baking pan with parchment paper, leaving an overhang on the sides to lift the blondies out later.

In a small bowl, mix together 1/4 cup granulated sugar and 1 tablespoon ground cinnamon. Set aside.

In a large bowl, whisk the melted butter, brown sugar, and the remaining 1/4 cup of granulated sugar until combined.

Whisk in the egg, egg yolk, and vanilla extract until the mixture is glossy and smooth, about 1 minute.

In a separate bowl, combine the flour, cream of tartar, baking soda, and salt. Add the dry ingredients to the wet ingredients and fold with a spatula until just combined. Do not overmix.

Spread the thick batter evenly into the prepared pan. Sprinkle the cinnamon-sugar mixture evenly over the top.

Bake for 22-26 minutes, or until the edges are golden and a toothpick inserted into the center comes out with moist crumbs. The center should be soft-set.

Let cool completely in the pan on a wire rack before lifting out and slicing into 16 squares.