Sift together almond flour, powdered sugar, and salt. Set aside. Beat egg whites until foamy, gradually add granulated sugar, and beat until stiff peaks form. Add vanilla extract and mix well.
Fold in the dry ingredients gently in three additions. Divide batter, color with gel food coloring, and pipe onto baking sheets.
Let piped macarons sit for 30-60 minutes to form a skin. Preheat oven to 300°F and bake for 15-18 minutes. Cool completely.
Prepare filling by beating softened butter, adding powdered sugar, vanilla extract, and milk until desired consistency is reached.
Assemble macarons by pairing shells, spreading filling on one shell, and sandwiching with another shell.