Mash raspberries through a fine-mesh sieve until you have 1 to 2 tablespoons of juice. Discard solids or reserve for another use. In a medium glass measuring cup, stir together buttermilk and raspberry juice.

In the bowl of a stand mixer fitted with the paddle attachment, beat butter and salt at low speed until smooth, 1 to 2 minutes. Slowly add one-fourth of confectioners’ sugar and one-fourth of buttermilk mixture, beating until smooth; continue alternately adding confectioners’ sugar and buttermilk mixture until frosting is smooth and fluffy. Scrape sides of bowl, and increase mixer speed to medium, beating for 5 minutes. Add food coloring (if using).