01 - 
                Beat softened butter and granulated sugar together in a mixing bowl until light and fluffy.
              
              
              
                02 - 
                Mix in the egg, egg yolk, and vanilla extract until smooth.
              
              
              
                03 - 
                Blend in baking soda, cream of tartar, salt, and flour until dough forms.
              
              
              
                04 - 
                Scoop 2-tablespoon portions of dough onto a baking sheet and bake at 350°F (175°C) for 11–15 minutes, until lightly golden at the bottom.
              
              
              
                05 - 
                Let cookies cool on the baking sheet for 10 minutes before transferring to a wire rack. Cool completely before frosting.
              
              
              
                06 - 
                Mix softened butter, powdered sugar, vanilla extract, salt, and milk until smooth. Add food coloring if desired.
              
              
              
                07 - 
                Frost cooled cookies and decorate with sprinkles as desired.