1️⃣ Preheat your oven: Preheat your oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, leaving some overhang for easy removal. Lightly grease the parchment paper.

2️⃣ Make the dough: In a large mixing bowl, cream the softened butter and sugar together until light and fluffy, about 2–3 minutes. Beat in the eggs, one at a time, followed by the vanilla extract. Mix until smooth. In a separate bowl, whisk together the flour, cream of tartar, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.

3️⃣ Prepare the cinnamon-sugar topping: In a small bowl, mix the granulated sugar and cinnamon until well combined.

4️⃣ Assemble the bars: Press half of the cookie dough evenly into the prepared baking pan. Sprinkle about 2 tablespoons of the cinnamon-sugar mixture evenly over the dough. Drop spoonfuls of the remaining dough over the cinnamon-sugar layer, gently spreading it to cover as much as possible. Sprinkle the remaining cinnamon-sugar mixture evenly over the top.

5️⃣ Bake the bars: Bake in the preheated oven for 25–30 minutes, or until the edges are golden brown and the center is set. A toothpick inserted into the center should come out mostly clean (a few crumbs are okay). Let the bars cool completely in the pan on a wire rack before slicing.

6️⃣ Slice and serve: Use the parchment paper overhang to lift the bars out of the pan. Cut into squares or rectangles and serve. Enjoy the gooey, cinnamon-sugar goodness!