8. Add softened butter to a medium bowl. Cream with an electric mixer until fluffy, then slowly add in powdered sugar free sweetener. Mix until combined. Add in vanilla extract and a pinch of salt. If buttercream is too thick for piping, add 1-3 tablespoons of nut milk or heavy cream. Stir in green and yellow food coloring until desired coloring is achieved. Place frosting in a piping bag fitted with a star tip.