Preheat the oven to 350°F / 175°C. Line a cookie sheet with parchment paper.
Add the butter to a small microwave safe mixing bowl. Microwave in 20-second intervals, stirring after each interval until the butter has just completely melted.
Stir in the brown sugar, 2 tsp. of milk, and vanilla.
Add in the flour, baking soda, and salt. Mix together just until a few flour streaks remain, then mix in the chocolate chips just until no flour streaks remain.
Transfer ½ of the cookie dough to another small mixing bowl. Add in the cocoa powder and remaining 1 tsp. of milk, and gently fold together just until the mixture is cohesive.
Use a 1 ½ tbsp. scoop to scoop each of the cookie doughs (You should end up with two of each flavor.)
Use a knife or your hands to split each ball of cookie dough in half. Alternate adding each half ball of flavor to your hand, for a total of four pieces of dough (ex. vanilla, chocolate, vanilla, chocolate)
Roll the dough into a ball, and repeat with the remaining cookie dough. (Tip: for uniform balls of dough, you can gently press each cookie dough ball into a 3 tbsp. cookie scoop!)
Set the cookie dough in the freezer to chill for 5 minutes.
Evenly space out the cookie dough balls on a parchment lined cookie sheet. Top the cookies with extra chocolate chips as desired. Bake for 11-13 minutes, until the edges start to turn golden brown and the middle of the cookie no longer looks “wet.”
Allow the cookie to rest on the hot baking sheet for 5-10 minutes before transferring it to a wire rack to cool.
Flip one of the cookies upside down and place a heaping scoop of ice cream on top of it. Sandwich with the other cookie, then enjoy!