Combine butter, granulated sugar, brown sugar, peanut butter, egg, milk, vanilla extract, flour, baking soda, and salt in a large bowl. Mix with an electric mixer until combined.
1/2 cup salted butter, 1/2 cup granulated sugar, 1/2 cup brown sugar, 1/2 cup creamy peanut butter, 1 large egg, 2 Tablespoons milk, 1 teaspoon vanilla extract, 1 3/4 cup all-purpose flour, 1 teaspoon baking soda, 1/2 teaspoon salt