In a medium bowl, whisk together the sifted powdered sugar and softened butter until the mixture is crumbly and begins to come together.
Use a hand mixer or whisk to beat the mixture on medium speed for about 2 minutes, until it becomes smooth, fluffy, and creamy.
Add the vanilla extract and a tablespoon of heavy cream or milk. Continue mixing on medium speed, scraping down the sides, until the frosting is silky and spreadable, about 1-2 minutes.
If you want colored frosting, add small amounts of gel food coloring, stirring until fully incorporated and vibrant. Add lemon zest if using, for a bright flavor.
Check the consistency: it should be creamy and firm enough to hold its shape. If it’s too soft, sprinkle in a little more powdered sugar; if too stiff, stir in a tiny splash of milk.
Transfer the frosting into piping bags for decorating or spread with a spatula onto cooled sugar cookies. Cover with plastic wrap if not used immediately to prevent drying.
Allow the frosting to rest at room temperature for 10-15 minutes to develop flavor and slightly firm up for piping or spreading.
Decorate your cooled cookies with piping tips or smooth spreads, creating swirls, stars, or gentle waves as you desire.
Once decorated, let the frosting set for about 30 minutes at room temperature before serving or storing.
