Preheat oven to 350F (175C) and line a baking sheet with parchment paper. Set aside.
In a large mixing bowl, use an electric mixer to beat together butter and sugar until light and fluffy.
½ cup unsalted butter, 1 cup granulated sugar
Add sour cream, egg, and vanilla extract and beat together until creamy and well combined. If using lemon zest, stir this in here as well.
½ cup sour cream, 1 large egg, 1 teaspoon vanilla extract, 1 Tablespoon fresh lemon zest
In a separate, medium-sized bowl whisk together flour, baking powder, baking soda, and salt.
2 ½ cups all-purpose flour, 1 teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon salt
Gradually add dry ingredients to wet, stirring together until completely combined.
Scoop dough by level 1 ½ Tablespoon sized scoop and drop onto prepared baking sheet, spacing cookies at least 2” (5cm) apart. Transfer to center rack of preheated 350F (175C) oven and bake for 10-11 minutes.
Within a minute of the cookies coming out of the oven, very lightly flatten the tops with the bottom of a clean glass or measuring cup (optional, but allows for a flat surface for icing). Allow to cool completely on the cookie sheet before frosting.
Frosting
Combine butter and sugar in a large mixing bowl and use an electric mixer on low-speed to beat together until smooth and creamy.
½ cup unsalted butter, 1 ½ cups powdered sugar
Add sour cream, vanilla extract, and salt and beat together to completely combine. If using food coloring, add a few drops and stir to evenly color the frosting.