Preheat the oven to 350°F.
In a small bowl, mix together the melted butter, brown sugar, white sugar, whisked egg, vanilla extract, baking soda, and salt until smooth.
Add the all-purpose flour and fold it in with a rubber spatula until you achieve a cookie dough consistency. If the dough is a bit too wet, add 1/2 to 1 tablespoon of flour as needed.
Mix in the milk chocolate and dark chocolate, then let the dough rest in the fridge for 15 minutes.
Line a sheet pan with parchment paper.
Roll the dough into a ball and place it on the lined sheet pan.
Optionally, sprinkle some sea salt on top.
Bake in the oven for about 8 minutes until the edges are golden brown and the center is still a bit soft. Avoid overbaking; the cookies will continue baking on the hot pan even after being taken out of the oven.
Leave the cookies on the pan for about 10 minutes or until firm before transferring them to a cooling rack to finish cooling. Now, savor the deliciousness!