If starting with cold butter, double the whipping time. If it is a little too thick, add 2 more teaspoons of milk and mix for 5 more minutes. You can store buttercream in the refrigerator for up to 1 week or freeze it for up to 1 month. When you are ready to use frozen buttercream, move it to the refrigerator the day before and then whip it in the stand mixer. If the buttercream looks “broken” or “split,” add 24–36 g (3–4 tablespoons) of powdered sugar as you whip it again to bring it back together. Also make sure the mixture is at room temperature when mixing. If using cold buttercream, it will take twice as long to come back together into a smooth mixture.