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How to thicken frosting
Add more powdered sugar.
Most frostings contain powdered sugar, and the most common way to thicken a runny frosting is by gradually mixing in a bit more powdered sugar to offset the liquid ingredients.
Only add the sugar a little at a time.
Sprinkle an additional 1 to 2 tbsp (15 to 30 ml) into the frosting and mix well. If you add too much at once, the frosting will become too sweet and may become too thick.