Preheat oven to 350°F and spray a 9x13" pan with cooking spray.
In the bowl of a stand mixer, add the butter, brown sugar and sugar; mix until light and fluffy.
Add the eggs and vanilla and mix until well-combined, scraping down the sides of the bowl halfway through.
Add the flour, baking soda and salt and mix until incorporated.
Add 1 cup of the semi-sweet chocolate chips (save the rest for the top) and the milk chocolate chips and stir.
Spread the cookie dough into the prepared 9x13" pan and smooth out the top.
Sprinkle remaining chocolate chips on top.
Bake for 30 minutes, or until the edges are a light golden-brown.
Let cool, then cut into squares and enjoy!