Follow these steps carefully for perfectly soft and chewy sugar cookie bars:

Making the Sugar Cookie Bars:

1. Preheat & Prepare: Preheat your oven to 375°F (190°C). Grease and flour a 9×13 inch baking pan, or preferably, line it with parchment paper, leaving an overhang on two sides. The parchment paper makes removal and cleanup much easier.
2. Cream Butter and Sugar: In a large mixing bowl, using either a stand mixer fitted with the paddle attachment or a hand mixer, beat the softened unsalted butter and granulated sugar together on medium-high speed for about 2-3 minutes. The mixture should become light in color, fluffy, and significantly increased in volume. This step incorporates air, which is vital for a tender texture.
3. Add Eggs and Vanilla: Add the large eggs one at a time, beating well after each addition until fully incorporated. Scrape down the sides and bottom of the bowl as needed to ensure everything is evenly mixed. Beat in the vanilla extract until combined.
4. Combine Dry Ingredients: In a separate medium bowl, whisk together the all-purpose flour, baking powder, and salt. This ensures the leavening agent and salt are evenly distributed throughout the flour, preventing pockets of saltiness or areas that don’t rise properly.
5. Combine Wet and Dry: Gradually add the dry ingredient mixture to the wet ingredient mixture in the large bowl. Mix on low speed, starting slowly to avoid flour flying everywhere, just until the flour streaks mostly disappear. Be careful not to overmix at this stage; overmixing develops the gluten in the flour, which can lead to tough, dense bars instead of soft and tender ones. Stop mixing as soon as you no longer see large patches of dry flour. A few small streaks are okay.
6. Press Dough into Pan: The dough will be thick and somewhat sticky. Transfer the dough into the prepared 9×13 inch baking pan. Using lightly floured hands, an offset spatula, or the back of a greased spoon, press the dough evenly into the bottom of the pan, ensuring it reaches all corners and has a relatively level surface. An even layer ensures even baking.
7. Bake: Place the pan in the preheated oven and bake for 20-25 minutes. The edges should be lightly golden brown, and a wooden skewer or toothpick inserted into the center should come out with moist crumbs attached, but no wet batter. Overbaking will result in dry bars, so start checking around the 20-minute mark. The center might look slightly underdone, but it will continue to cook slightly as it cools in the pan.
8. Cool Completely: Remove the pan from the oven and place it on a wire rack. Let the sugar cookie bars cool completely in the pan before frosting. This is crucial! Attempting to frost warm bars will result in melted, messy frosting. Cooling can take 1-2 hours at room temperature.

Making the Creamy Buttercream Frosting:

1. Beat the Butter: In a large mixing bowl (you can reuse the one from the cookie dough after cleaning it), beat the softened unsalted butter using a stand mixer with the paddle attachment or a hand mixer on medium-high speed until it’s very smooth and creamy, about 2 minutes.
2. Add Powdered Sugar Alternately with Liquid: Gradually add the sifted powdered sugar, about one cup at a time, alternating with the heavy cream or milk (start with 3 tablespoons). Begin mixing on low speed after each addition to prevent a sugar cloud, then increase to medium speed and beat well. Continue adding powdered sugar and liquid until all the sugar is incorporated and the frosting reaches your desired consistency. If it’s too thick, add a tiny bit more cream/milk (a teaspoon at a time). If it’s too thin, add a bit more sifted powdered sugar (a tablespoon at a time).
3. Add Flavor and Beat Until Fluffy: Add the vanilla extract and a pinch of salt. Beat the frosting on medium-high speed for another 2-3 minutes until it’s light, fluffy, and very smooth. This incorporates air, making the frosting exceptionally creamy.
4. Add Color (Optional): If using food coloring, add a few drops now and mix on low speed until the color is evenly distributed. Add more color gradually until you achieve the desired shade. Gel food coloring provides more vibrant color with less liquid.

Frosting and Finishing:

1. Frost the Cooled Bars: Once the sugar cookie bars are completely cool, spread the prepared buttercream frosting evenly over the top using an offset spatula or the back of a spoon.
2. Add Sprinkles: Immediately sprinkle generously with your chosen sprinkles while the frosting is still wet, so they adhere properly.
3. Cut and Serve: If you used parchment paper, carefully lift the entire slab of bars out of the pan using the overhangs and place it on a cutting board. Cut into desired sizes (squares or rectangles – typically 12, 16, or 24 bars depending on preference). If you didn’t use parchment, cut the bars directly in the pan. Wipe the knife clean between cuts for neat edges.