Velvet Sugar Cookie Squares with Silken Buttercream

Ingredients

• 1 tablespoon pure vanilla extract
• 1½ cups of butter, softened
• 4½ cups of all-purpose flour
• ½ cup of butter, softened
• ½ teaspoon of salt
• 2 large eggs
• 1½ cups of granulated sugar
• 1 teaspoon cream of tartar
• 5 cups of powdered sugar
• 1 teaspoon baking soda
• 1/3 cup of whole milk