01 -
Beat softened unsalted butter and granulated sugar in a large bowl until the mixture is pale and fluffy, incorporating air to ensure tender cookies.
02 -
Blend in the large egg, egg yolk, and vanilla extract until the mixture is smooth and fully combined.
03 -
Gradually add baking soda, cream of tartar, salt, and all-purpose flour, mixing on low speed just until a dough forms to avoid toughening.
04 -
Scoop out approximately 2-tablespoon portions of dough and place them spaced apart on a lined baking sheet to allow spreading.
05 -
Bake in a preheated oven at 350°F (175°C) for 11 to 15 minutes, until edges are lightly golden and centers remain soft. Let cool on the baking sheet for 10 minutes before transferring to a wire rack.
06 -
Beat softened butter, powdered sugar, vanilla extract, and salt until creamy. Gradually add milk until smooth and tint with food coloring if desired.
07 -
Spread the buttercream evenly over completely cooled cookies and garnish with sprinkles to your preference.