Place the 2 cups of cream soda for the buttercream in a saucepan bring to a boil and simmer until the cream soda is reduced to approximately 1/2 cup. Pour into a mason jar and place in the fridge to cool.
Place the butter and icing sugar in a large mixing bowl and beat together until completely combined. Add 2 Tablespoons of the reduced cream soda syrup, then taste and adjust with more cream soda syrup.