Using a food processor or blender, crush the Fruity Pebbles cereal. Set aside.
Put sticks of butter in electric mixer and mix until butter is fluffy and light.
Add in cream, Fruity Pebbles, and vanilla. Mix on low until well combined, making sure to scrape the sides of the bowls.
Mix in 1 cup of powdered sugar at a time until well combined. Turn mixer on high for 2 minutes to get the buttercream nice and fluffy.
Frost cooled cookies and top with Lucky Charms cereal!