You can do this by lightly coating it with a nonstick cooking spray or by covering it with parchment paper. You can do this using an electric mixer or a food processor fitted with whisks. You can also do it by hand using a whisk, but it will take much longer. Keep mixing until the sugar is mixed evenly throughout the butter.  If you are vegan, you can use "vegan butter" instead. If the butter is too hard, let it sit at room temperature until it softens, or microwave it for about 45 seconds (depending on the strength of your microwave). Do not melt the butter. Keep mixing until everything is evenly combined. If you are vegan, you can use a non-dairy milk instead, such as almond or soy. Mixing the dry ingredients separately will ensure that everything mixes evenly together later on. Eventually, the ingredients will start to clump together to form a dough. If you are using a whisk, you might find it helpful to switch to a rubber spatula. If the flour is too sticky, add up to 3 tablespoons (22.5 grams) of flour. All that mixing and whisking likely made the dough very soft and sticky. This will make it hard to work with when you roll it out. Wrap the dough up with plastic wrap, then stick it in the fridge until it hardens by up again, about 30 minutes. You can also use a tablespoon or an ice cream scoop to make the balls. You have enough dough to make 12 of them. Do not roll the dough out into a flat sheet and cut out shapes using cookie cutters; this is the wrong type of dough for that type of cookie. You want the balls to be about ½ to ⅔-inch (1.27 to 1.7 centimeters) thick. They will continue to flatten and spread as they are baking. A simple way to do this is to fill a shallow dish with 2 tablespoons (30 grams) of sugar, and then pressing one side of each cookie into it. Do not coat both sides of the cookies. Also, make sure that there are a few inches/centimeters of space between each cookie; if the cookies are too close, they will mush together. For a more colorful cookie, use colored sugar or nonpareil sprinkles. The cookies are ready when the edges turn a light, golden color. Be sure to use a spatula to transfer the cookies so that you don't burn yourself or break them. Once the cookies are completely cool, you can frost them or serve them as they are.

Summary:
Preheat your oven to 350°F (280°C) and prepare your baking sheet. Beat the butter until it becomes light and fluffy, then mix in the sugar. Whisk the vanilla extract and milk into the butter mixture. Whisk together the salt, flour, and baking powder in a separate bowl. Add the flour mixture into the butter mixture, and stir until evenly combined. Chill the dough for 30 minutes in the fridge. Roll the dough into small balls. Flatten the balls a little using your palm or the bottom of a glass. Coat the cookies with sugar, then transfer them to a baking sheet. Bake the cookies for 8 to 10 minutes. Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire cooling rack.