Step 1

Preheat oven to 350 degrees.

Step 2

Place 1/2 cup of the chocolate chips in a plastic bag and use a rolling pin to break them down into small pieces (set the other 1/2 cup of unchopped chocolate chips aside). The goal here is to have a mixture of tiny chocolate flakes throughout the dough, while still have the whole chunks in there for texture. Once chopped, set aside.

Step 3

In a small microwave-safe bowl, melt the butter for 20-30 seconds until JUST melted.

Step 4

Add in white sugar and brown sugar and use a spatula to stir until combined (* do not use a mixer!). Add in egg and vanilla, then mix. Next, add in baking soda, baking powder, and salt and mix again. Add in flour and mix until all the flour disappears into the dough.

Step 5

Add the 1/2 cup of chopped chocolate chips and 1/2 cup of whole chocolate chips into the dough and mix.  Immediately proceed to pressing into pan (you don't want the dough sitting out or it will set too much with the melted butter).

Step 6

Grease a 11.5" metal cookie cake pan (don't have one? See notes below.) Although this cookie removes cleanly, if you are worried about it sticking, line the bottom and sides of the pan with parchment paper first. Using a spoon or your clean hands, drop the dough into small chunks in the pan. This will help disperse everything so that you can easily smooth it out. Once all dough as been added, use a spatula or your hands to press down the dough into a smooth layer (being sure to get all the corners).Tip: If you notice 2-3 chocolate chips are stuck together, separate them before baking. This will allow the chocolate to be evenly dispersed and prevent the cookie from having a giant clump of chocolate in one spot (which could cause it to more easily break while cutting).

Step 7

Bake at 350 degrees for 12-14 minutes - do NOT overbake or your chocolate chip cookie cake will not be soft. You want the cookie cake to be slightly brown on the top and look slightly underbaked (my oven was perfect at 12 minutes). Remove from oven and place entire pan on a cooling rack. Allow the cookie cake to cool completely before removing.Tip: To remove from pan, use a knife to go around the edges, place a flat platter on top, then flip over the pan to remove. If you used parchment paper, this should come out rather smoothly. After flipping over the cookie cake, flip it over again on the final decorate plate or platter (I LOVE these cute paper trays and the largest one in the pack is perfect for a 10" cookie cake).

Step 8

Using a large star piping tip, pipe around the edges then immediately add sprinkles (the frosting will start hardening quick, so add sprinkles right away!).If you're feeling fancy, you can double the frosting recipe to have enough to pipe phrases like "Happy Birthday" on the top. Or use this fool-proof letterboard cake topper kit (these are made from plastic so you can wash and reuse for years!).

Step 9

Beat the softened butter on medium-high speed for 2-minutes until smooth. Turn the mixer to low speed then add the powdered sugar 1/4 cup at a time. You may need to stop and scrape down sides. Add vanilla and salt and turn mixer to medium speed. One tablespoon at a time, add milk. Beat on high for 2-3 minutes until creamy and the frosting is at a good consistency to pipe. If it looks too runny, add more powdered sugar. If it looks too stiff, add a tiny bit of milk until you get to the desired consistency.This will make just enough frosting to pipe a border along the edges. If you would like more frosting to cover the top or to write words on top, simply double the frosting recipe!