Start by preheating your oven to 350°F. Lightly spray a standard muffin tin with cooking spray and set aside.

Melt the butter and allow it to cool slightly.

In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth. Add the egg and continue mixing until the mixture becomes creamy.

Add the flour, baking soda, and salt, stirring until a soft cookie dough forms. Fold in the chocolate chips until evenly distributed throughout the dough.

Using a standard cookie scoop (about 2 tablespoons of dough), portion the dough into the prepared muffin tin. Gently press each dough ball into the cavity, bringing the dough slightly up the sides to create a shallow cookie cup.

Bake for 9 to 11 minutes, or until the edges are lightly golden and the centers are set.

Allow the cookies to cool in the muffin tin for about 10 minutes, then carefully loosen each cookie with a small offset spatula or butter knife before transferring them to a wire rack to cool completely.

Meanwhile, prepare the buttercream. Using an electric mixer, beat together the softened butter, powdered sugar, and milk until light, smooth, and spreadable.

Once the cookies have cooled completely, spread a layer of buttercream over the top of each cookie, allowing it to fill the shallow center.

Pour the sprinkles into a shallow bowl. Gently press the buttercream side of each cookie into the sprinkles until completely coated.

Serve immediately or store in an airtight container until ready to enjoy.