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Use a spatula to transfer the frosting from the container to a mixing bowl. Add 1 tablespoon (15 g) of powdered sugar to the bowl and mix it by hand or with an electric mixer.  If you would like the frosting to be thicker, mix another ½ tablespoon (7.5 g) of powdered sugar into the frosting. Put the frosting in a mixing bowl using a spoon or spatula. Add ½ teaspoon (2.5 mL) of milk to the bowl. Mix it with an electric mixer or by hand. If the frosting is still too thick, mix another ½ teaspoon (2.5 mL) of milk into it. You can substitute milk for water, if desired. Transfer the frosting to a large mixing bowl. Whip the frosting using a whisk or an electric mixer until it doubles in volume. Don’t continue whipping after the volume has doubled, or you risk creating lumps in the frosting.

Summary:
Add 1 Tbsp of powdered sugar to thicken the frosting. Thin the frosting using ½ tsp of milk. Whip the frosting to make it light and fluffy.