📖 Instructions

1. **Preheat Oven and Prep Pan:** First off, get your oven ready! Preheat it to 350°F (175°C). While it's heating up, grab a 9x13 inch baking pan. You'll want to line it with parchment paper, making sure to leave an overhang on the sides. This makes lifting the cooled squares out super easy, trust me. If you don't have parchment, grease and flour the pan really well.
2. **Combine Dry Ingredients:** In a medium bowl, whisk together your all-purpose flour, baking soda, cream of tartar, and a pinch of salt. Give it a good whisk to ensure everything is evenly distributed. This step is important for an even rise and texture, so don't just dump it all in!
3. **Cream Wet Ingredients (Cookie Dough):** In a large bowl, using an electric mixer (or a strong arm and a whisk!), cream together the softened unsalted butter and granulated sugar until the mixture is light and fluffy. This usually takes about 2-3 minutes. Next, beat in the large egg and the vanilla extract until everything is well combined and smooth. Don't overmix here, just get it incorporated.
4. **Combine Wet and Dry:** Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix the dough, as this can make your cookies tough. Once you see no more streaks of flour, stop mixing.
5. **Press Dough into Pan:** Evenly press about two-thirds of the cookie dough into the bottom of your prepared 9x13 inch baking pan. You can use your hands, a spatula, or even the bottom of a glass to get an even layer. This will be the base of your squares.
6. **Prepare Cinnamon Filling:** In a small bowl, melt the unsalted butter for the filling. Stir in the light brown sugar and ground cinnamon until it forms a thick, spreadable paste. If it feels a little too stiff, you can add a tiny splash of milk or cream, but it should be pretty thick.
7. **Spread Filling:** Carefully and evenly spread the cinnamon filling over the layer of cookie dough in the pan. Try not to disturb the bottom dough too much. A small offset spatula or the back of a spoon works wonders here.
8. **Add Remaining Dough:** Now, take the remaining one-third of the cookie dough. Dollop small pieces of this dough over the cinnamon filling. Don't worry about covering the entire surface perfectly; it's okay if some of the cinnamon filling peeks through. As it bakes, the dough will spread and melt into a beautiful, gooey top.
9. **Bake the Squares:** Pop the pan into your preheated oven and bake for 25-30 minutes. You're looking for the edges to be lightly golden brown and the center to be just set, but still looking soft and slightly gooey. If it looks too wet in the middle, give it another 2-3 minutes, but keep an eye on it! Oven temperatures can vary, so adjust as needed.
10. **Prepare Glaze:** While the squares are baking or cooling, make your glaze. In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until you have a smooth, pourable glaze. If it's too thick, add a tiny bit more milk; if too thin, a bit more powdered sugar. You want it drippy but not watery.
11. **Cool and Glaze:** Once baked, remove the pan from the oven and let the cookie squares cool in the pan on a wire rack for at least 15-20 minutes. They need this time to set up and become truly gooey. Once slightly cooled but still warm, drizzle the prepared glaze generously over the top. The warmth will help the glaze spread beautifully.
12. **Slice and Serve:** Allow the squares to cool completely before lifting them out of the pan using the parchment paper overhang. Slice into squares and serve! Seriously, try to resist eating them all at once. They're best enjoyed fresh, maybe with a glass of milk or a cup of coffee. Enjoy your delicious, homemade gooey cinnamon cookie squares!