Add nuts to a food processor and blend into a meal. Remove and set aside.
Add powdered sugar 1 Tbsp at a time until you reach desired sweetness. I found that 5-6 was perfect for me, plus helped it thicken up a bit (amount as original recipe is written).
At this point it's perfect for adding to a squeeze bottle or piping bag and piping onto your baked good. Otherwise, you can refrigerate it to make it thicker. Will keep stored in the refrigerator for up to a month. Recipe (as originally written) makes enough to frost at least 3-4 dozen cookies + leftovers.
*Nutrition information reflects 1 of about 50 servings. This is just an estimation.
*If you can't find or don't like macadamia nuts, sub a similar nut such as cashews.
*Make chocolate frosting by adding ~ 1/2 cup melted bittersweet chocolate.