Preheat oven to 350°F. Spread flour on a parchment lined or silicone baking mat lined rimmed baking sheet.: Even though you will not be baking cookies, bringing your oven to 350°F ensures the internal temperature is stable when you toast the all purpose flour . You should notice the oven warming and a faint dry heat; if your oven has hot spots, rotate the sheet once while the flour toasts. Common mistake to avoid, do not leave the flour unattended, because it can brown quickly and become bitter.
Bake the flour for about 6 minutes and then set it aside to cool. The flour needs to be completely cooled before proceeding. (Check the flour to make sure it hits 160 degrees to be safe.) You do not want the flour to brown so keep a close eye on it.: When you spread the flour in an even layer, you promote consistent toasting, which gives a uniform, toasted aroma and prevents pockets of raw flour. Use the back of a spoon to level it, and the layer should be thin enough that you can see a faint dusting across the sheet. A frequent error is piling the flour too thickly, which leads to uneven heating and longer bake times.
In a large mixing bowl or the bowl of a stand mixer, cream together the butter and sugar until light and fluffy, about 3 to 4 minutes.: As the flour toasts, you will smell a warm, biscuity scent that is more fragrant than raw flour, and the surface should remain pale, not golden brown. Let it cool fully to room temperature so it does not melt the butter later, and for safety verify it reaches 160 degrees if you have a thermometer. A typical pitfall here is adding warm flour to the butter, which will make the dough greasy instead of creamy.
Mix in the vanilla extract, cooled flour, salt and 2 tablespoons of milk. Mix on low speed just until the mixture starts to come together in a ball. Add additional milk if the dough appears dry or crumbly.: As you beat the salted butter with the granulated sugar , the mixture should become paler and expand slightly as air incorporates; you will hear a softer, smoother sound from the mixer and see a ribboning texture when you lift the paddle. This aeration is what gives the dough a lighter mouthfeel. A common error is using butter that is too cold, which prevents proper creaming and yields a dense texture.
Stir in sprinkles if using.: Once you fold in the vanilla extract and the cooled, baked all purpose flour , the aroma will bloom and the mixture will lose its glossy sheen as dry ingredients absorb moisture. Start on low to avoid a flour puff, then increase briefly if needed to bring the dough together. If it looks crumbly, add the extra tablespoon of whole milk slowly, watching for the dough to form a cohesive, slightly tacky ball. Avoid over mixing, which can make the dough dense and cause it to lose its tender quality.
Store in an airtight container in the refrigerator for up to 3 days. Dough may need to sit out at room temperature for 10 minutes before soft enough to eat with a spoon.: Sprinkles bring a cheerful texture contrast and visual pop, and they will disperse evenly with gentle folding; you should see flecks of color without streaks. Fold them in by hand or on low speed to preserve their shape and avoid creating a colored streak through the entire dough. One mistake to watch for is adding too many mix ins, which can overwhelm the delicate sugar cookie taste.
Store in an airtight container in the refrigerator for up to 3 days. Dough may need to sit out at room temperature for 10 minutes before soft enough to eat with a spoon.: Chilling lets the flavors meld and firms the dough for scooping or spooning. When refrigerated you will notice it becomes denser and cooler to the touch; let it rest on the counter for about 10 minutes to soften for easier serving. A common error is letting it sit too long unrefrigerated, which can promote spoilage, so always use an airtight container and keep to the time window.