With a stand mixer or a handheld beater, beat the butter until it is light and fluffy, 1-2 minutes.
Add both sugars and beat again for another 2 minutes. Add the eggs and vanilla and beat until incorporated. Scrape down the sides where needed.
Mix the flour, baking soda & salt together and add them into the mixer on a low speed until just combined. Add the chocolate chips and fold them in gently with a spatula.
Add the cookie mixture to an 8' greased cake tin or pie tin, pressing it flat with your hands. Top with a handful of sprinkles.
Bake at 175C for 20 minutes. Once cooked a toothpick should come out clean. If the cookie is browning too much, cover with foil while it's still in the oven.
Once the cookie has cooled, beat all the frosting ingredients together until you have a pipeable thickness. You may need more/less milk so start with 1 tablespoon.
Pipe spirals around the edges and top with sprinkles.
