Add 8 ounces unsalted butter (softened) and 2 teaspoons pure vanilla extract to medium mixing bowl. Using hand mixer with beater attachments or stand mixer with paddle attachment, beat butter and vanilla extract together on medium-low speed until fully combined, approximately 1 to 2 minutes. Scrape down sides of bowl with silicone spatula as needed.
Sift 16 ounces powdered sugar into bowl in increments, mixing on low speed after each addition until sugar is incorporated into butter. Once all sugar has been added, add ¼ teaspoon salt and mix until incorporated.
Scrape down sides of bowl with silicone spatula as needed. Set mixer to medium speed and add 4 tablespoons whole milk in 1-tablespoon increments, fully incorporating each tablespoon before adding more, until desired consistency is achieved. If needed, mix in additional powdered sugar to thicken frosting.
Optional: If using food coloring, add desired amount of coloring and mix until fully incorporated, adding more coloring as needed to achieve desired results.
Transfer prepared frosting into piping bag and pipe onto completely-cooled cookies. Alternately, use offset spatula to dollop small amounts of frosting onto each cookie and spread frosting over cookie to cover.
If desired, top frosted cookies with sanding sugar, shaped sprinkles, nonpareils, jimmies, edible glitter, or other decorations of choice. Let frosted cookies stand 3 to 5 minutes or until buttercream frosting is slightly set, then serve cookies as desired.