📖 Instructions

1. Preheat your oven to 350°F (175°C). This ensures the oven is ready and at the correct temperature when you place your cookie squares inside. Line an 8x8 inch (20x20 cm) baking pan with parchment paper, leaving an overhang on two sides to easily lift the bars out later. A little spray of cooking oil underneath the parchment can help it stick to the pan.
2. In a large mixing bowl, beat together the softened unsalted butter and granulated sugar using an electric mixer (or a sturdy whisk if you're feeling strong!) on medium speed. You're aiming for a mixture that's light, fluffy, and pale in color, which usually takes about 2-3 minutes. Make sure to scrape down the sides of the bowl with a spatula periodically to ensure everything is well combined.
3. Beat in the large egg and pure vanilla extract. Continue mixing until everything is fully incorporated and the mixture is smooth. Don't overmix here, just enough to combine.
4. In a separate, medium-sized bowl, whisk together the all-purpose flour, baking soda, and salt. Make sure there are no lumps in the flour mixture. This step is important for an even texture.
5. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix the dough, as this can lead to tough cookies. As soon as you see no more streaks of flour, stop mixing.
6. Press about two-thirds of the cookie dough evenly into the bottom of your prepared 8x8 inch baking pan. You can use your hands, a spoon, or a small offset spatula for this. Try to get it as even as possible for uniform baking.
7. Now, for the glorious cinnamon swirl! In a small bowl, combine the brown sugar, ground cinnamon, and the remaining 2 tablespoons of melted unsalted butter. Mix these together until you have a thick, paste-like mixture.
8. Dollop spoonfuls of the cinnamon mixture over the pressed dough in the pan. Don't worry about covering every inch; you want little pockets of cinnamon goodness. Then, take the remaining one-third of the plain cookie dough and drop small, roughly flattened pieces over the cinnamon mixture. It doesn't need to be perfect coverage—a rustic, marbled look is what we're going for!
9. Using a knife or a skewer, gently swirl the cinnamon mixture into the dough. Create some nice patterns, but again, don't overdo it. Just enough to create those lovely ribbons of cinnamon.
10. Bake for 22-28 minutes, or until the edges are lightly golden brown and a toothpick inserted into the center (avoiding the cinnamon swirls) comes out with moist crumbs, but not wet batter. The center might still look a little soft, which is perfect for that gooey texture. Keep an eye on it after 20 minutes; ovens vary!
11. While the cookie squares are baking, prepare your vanilla glaze. In a small bowl, whisk together the powdered sugar, milk (or heavy cream for a richer glaze), and 1/2 teaspoon of pure vanilla extract until smooth. If the glaze is too thick, add a tiny bit more milk, a quarter teaspoon at a time, until you reach a pourable but still thick consistency.
12. Once baked, remove the pan from the oven and let it cool in the pan on a wire rack for at least 15-20 minutes before drizzling with the vanilla glaze. This cooling time allows the bars to set up a bit. You can speed this up slightly by placing the pan in the fridge for 10 minutes, if you're impatient like me sometimes!.
13. After cooling slightly, drizzle the prepared vanilla glaze generously over the warm cookie squares. Let the glaze set for another 10-15 minutes, or until it’s firm enough not to smudge. Then, using the parchment paper overhangs, lift the entire slab out of the pan and transfer it to a cutting board.
14. Slice the cookie squares into 9 or 16 even pieces, depending on how big you like 'em. Serve warm or at room temperature. These are absolutely heavenly with a scoop of vanilla bean ice cream or a hot cup of coffee. Enjoy your delicious, homemade gooey cinnamon cookie squares!