Snickerdoodle Bars

Heat the oven to 350°F (175°C). Grease a 9 × 13 in (23 × 33 cm) metal baking pan and line it with a parchment sling overhanging the long sides. In a stand mixer with the paddle (or with a hand mixer), beat the softened butter with the granulated and brown sugars on medium speed for 2–3 minutes until lighter in color and fluffy.

Beat in the eggs one at a time, then the vanilla, scraping down the bowl. Whisk the flour, cream of tartar, baking soda, and salt in a separate bowl, add to the butter mixture, and mix on low speed just until no dry flour remains. The dough will be thick and soft, like cookie dough.

Spread the dough evenly into the pan with an offset spatula or damp fingers. Stir the topping sugar and cinnamon together and sprinkle it in an even layer over the whole surface. Bake 22–28 minutes, until the edges are light golden and pulling away from the pan and the center is puffed and set but still soft; a toothpick in the center should come out with a few moist crumbs, not wet dough.

Cool in the pan on a wire rack for at least 1 hour so the center finishes setting. Lift out with the parchment sling and cut into 24 bars with a sharp knife, wiping the blade between cuts.