Place the powdered sugar and softened butter into a mixing bowl and whisk together on low until the mixture is crumbly and begins to come together.
Add the milk and vanilla extract to the bowl, then whisk on low to start blending, gradually increasing to medium speed until the frosting becomes smooth and glossy, about 2 minutes.
Check the consistency; it should be soft and spreadable. If it's too thick, add a tiny splash more milk and whisk again. If too runny, sprinkle in a little more powdered sugar and fold in gently.
Whisk the frosting on medium speed for another 30 seconds to incorporate air, making it fluffy and easier to spread or pipe.
If desired, add gel food coloring a small drop at a time, folding it in thoroughly until you achieve your preferred shade.
Once colored, use a spatula or butter knife to spread the frosting evenly over cooled sugar cookies or pipe it for decorative designs.
For best results, chill the decorated cookies for 15-20 minutes to let the frosting set and firm up slightly, making them easier to handle.
Enjoy your beautifully frosted cookies immediately or store them in an airtight container at room temperature for up to 2 days. For longer storage, refrigerate and bring to room temp before serving.
