Place melted butter in a large bowl (or bowl of a stand mixer fitted with the paddle attachment). Add granulated sugar and brown sugar and mix on low speed until the mixture is smooth. Mix in egg and vanilla extract and mix on medium speed until combined.

Mix in salt and baking soda, then slowly mix in flour and mix just until the batter is smooth and comes together. Be sure to scrape the sides of the bowl during mixing. Slowly mix in chocolate chips.

Press batter into prepared pan. Bake for about 21-25 minutes or until the cookie is just slightly golden brown on top and a toothpick comes out clean from the center. Let cool before frosting.

Add butter to a large bowl and beat until smooth with a hand mixer. Add powdered sugar, cocoa and salt and mix until crumbly, them mix in vanilla extract.

Add 1 tablespoon milk or heavy cream and mix until smooth, adding more cream as needed for piping consistency.

To assemble the cookie cake: place frosting in a piping bag fitted with a large star tip and pipe swirls around the edges. Add sprinkles and serve.

Store in an airtight container for up to 3 days or freeze unfrosted cake for up to a month.