In a mixing bowl, start by creaming the butter and vegetable shortening until it becomes smooth. Use the lowest setting on your mixer.
1/4 cup (56g) salted butter, 1/4 cup (48g) vegetable shortening
Add one cup of powdered sugar, followed by just enough milk or heavy cream to blend the powdered sugar and butter mixture together.
3 cups (321g) powdered sugar, 1/4 cup (60mL) heavy cream
Repeat this step for the remaining powdered sugar, continuing to mix on the lowest setting. At this point, the frosting should be quite thick.
Now, add the vanilla extract (or flavoring of your choice) and mix thoroughly into the frosting.
2 tsp clear vanilla extract
To achieve your desired frosting consistency, gradually add small amounts of milk or heavy cream at a time. Keep the mixer on the lowest setting while doing this.
Once you've reached the ideal consistency, divide the frosting into separate bowls and add gel food coloring to achieve your desired colors.
Transfer the colored frosting into piping bags for decorating the cooled cookies, or you can use a knife to spread it onto the cookies.