Add softened Grassland butter into a standard mixer and beat until light. Add powdered sugar and mix on low until incorporated.
Slowly add in eggnog, along with the nutmeg and rum extract, then increase speed to medium-high and beat until light and creamy. If needed, add additional eggnog for buttercream consistency.
Transfer the mixture to a piping bag to add to your favorite treat. We tried homemade Italian pizzelles with macadamia nuts.