Preheat the oven to 350F. Butter or spray a 9-inch round cake pan with baking spray.

In a medium bowl, sift together flour, baking soda, and salt.

In a large mixing bowl, beat butter until smooth. Add both of the sugars and beat on medium speed until smooth and well combined, about 2 minutes. (You don’t want to beat the mixture until very light and fluffy like you would for a cake because the air will make the cookie too crumbly and puffy.)

Add the egg and vanilla. Beat until well combined, scraping down the sides of the bowl as needed. While mixing on low speed, add the flour and beat just until combined. Fold in the chocolate chips by hand using a silicone spatula.

Press the cookie dough into an even layer in the prepared cake pan. Sprinkle with more chocolate chips if desired.

Bake for 20 to 25 minutes or until the cookie is lightly browned and appears dry on the surface. Let cool completely in the pan on a wire rack.

In a mixing bowl, beat butter until smooth. Add the powdered sugar, cocoa powder, milk, and vanilla. Beat on low speed until combined, about 2 minutes. Increase speed to medium and beat until light and fluffy, about 2 minutes. Transfer to a piping bag fitted with a decorative tip.

Remove the cookie from the pan and place on a serving plate. Pipe frosting around the edge of the cookie. Decorate with sprinkles, if desired.