• Preheat oven to 350℉.
• Place 1 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 cup of unchopped chocolate chips aside).
• In a small microwave-safe bowl, melt the unsalted butter (1 cup, i.e. 2 sticks) for 20-30 seconds until JUST melted.
• Add in granulated sugar (½ cup) and light brown sugar (1 cup) and use a spatula to stir until combined (* do not use a mixer!). Add in eggs (2 large) and vanilla extract (1 tablespoon), then mix. Next, add in baking soda (1 teaspoon), baking powder (½ teaspoon), and salt (½ teaspoon) and mix again. Add in all-purpose flour (3 cups) and mix until all the flour disappears into the dough.
• Add the 1 cup of chopped chocolate chips and 1 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).
• Grease a 13x18" half sheet metal cookie cake pan (see notes below for alternate pans. Although this cookie removes cleanly, if you are worried about it sticking, line the bottom and sides of the pan with parchment paper first. Press dough into an even layer in pan.
• Bake at 350℉ for 16-18 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 16 minutes). Remove from oven and place entire pan on a cooling rack. Allow the cookie cake to cool completely before removing.
• 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!).

To Make Frosting:

• Beat the softened unsalted butter (¾ cup, i.e., 1 ½ sticks) on medium-high speed for 2 minutes until smooth. Turn the mixer to low speed then add the powdered sugar ¼ cup at a time (3 ¾ cups total). You may need to stop and scrape down sides. Add vanilla extract (1 tablespoon) and salt (¼ teaspoon) and turn mixer to medium speed. One tablespoon at a time, add milk (3 tablespoons total). 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, as well as write a phrase on top.