Preheat oven to 350°F.
In a small bowl, add the melted butter, brown sugar, white sugar, whisked egg, vanilla extract, baking soda, and salt. Combine together until smooth.
Add the all-purpose flour and fold in with a rubber spatula until you have a cookie dough consistency. If the dough is a bit too wet, add 1/2 a tablespoon to 1 tablespoon of flour as needed.
Mix in the milk chocolate and dark chocolate and let the dough rest for 15 minutes in the fridge.
Line a sheet pan with parchment paper.
Roll the dough into a ball and place on the lined sheet pan.
Optional, add a sprinkle of sea salt on top.
Pop in the oven to bake for about 8 minutes until the edges are golden brown and the center is still a bit soft. Do not overbake - the cookies will continue baking out of the oven on the hot pan.
Leave them on the pan for about 10 minutes or until firm before transferring them to a cooling rack to finish cooling and enjoy!