Preheat oven to 350 degrees F and grease an 8-inch or 9-inch pie plate or cake pan with non-stick cooking spray.
In a large mixing bowl, beat together butter, brown sugar, and granulated sugar for 2 minutes. Then add the egg, egg yolk, and vanilla and mix until smooth.
Add flour, baking soda, and salt to a medium bowl. Mix to combined. Add dry ingredients to wet ingredients and beat until cookie dough forms.
Use a wooden spoon to mix in all but 2 tablespoons of chocolate chips. Press the cookie dough into your prepared pan. Sprinkle the remaining 2 tablespoons chocolate chips on top and press them down slightly.
Bake for 22-25 minutes. Let the cookie cool completely in the pan. If it's not cool the frosting will melt. (If you're not frosting the cake, you can serve it warm after letting it cool for 10 to 15 minutes.)
To remove your cake from the pan, run a sharp knife along the edge to loosen. Use a large spatula (or two!) to transfer the cookie cake to a platter.
Beat the butter, cocoa powder, powdered sugar, vanilla, and milk together in a mixing bowl until smooth. (see note)
Place frosting into a piping bag fitted with a 1M tip. Add sprinkles on top of the cake. Pipe on frosting along the edges of the cake in rosettes. Add a chocolate chip onto each rosette, if desired.
To adjust frosting consistency:
Add 1 teaspoon of milk and then mix to thin the frosting. Repeat as needed.
Add 1 tablespoon of powdered sugar and mix to thicken the frosting. Repeat as needed.