Prepare your pan

Grease an 8-inch (20 cm) square baking pan with butter or non-stick spray. For easy blondie removal, line it with parchment paper, allowing some overhang to lift the bars out later.

Mix the base

In a medium bowl, cream together the butter, ½ cup granulated sugar, dark brown sugar, egg, and vanilla until smooth. Stir in the salt, then gradually fold in flour, cream of tartar, baking powder, and nutmeg until a uniform chewy blondie batter forms.

Swirl in cinnamon sugar

Combine the remaining 3 tablespoons granulated sugar and 2 teaspoons ground cinnamon in a small bowl. Dollop half of your blondie batter into the prepared pan, sprinkle half of the cinnamon-sugar mixture over it, then layer the remaining batter. Finish by sprinkling the rest of the cinnamon-sugar blend on top.

Bake to golden perfection

Preheat your oven to 350 °F (175 °C). Bake for 22–25 minutes, until the edges are lightly golden and the center is set but still soft. A toothpick inserted near the center should come out with moist crumbs—not raw batter.

Cool and slice

Allow the blondies to cool completely in the pan—on a wire rack if possible—before lifting them out and slicing into squares. For clean, sharp edges, use a sharp knife and wipe it between cuts.