Melt the butter in a small bowl in the microwave, heating in 8-second increments.
Add the white and brown sugar, whisking for 1-2 minutes until well combined.
Add half of an egg yolk and whisk again until smooth.
Sift in the flour, baking soda, and salt, gently folding until almost combined.
Add the chopped chocolate and fold until evenly distributed.
Chill the dough in the freezer for 10-15 minutes.
Preheat the oven to 375°F and line a small baking sheet with parchment paper.
Form the chilled dough into a ball and place it on the baking sheet.
Bake for 10-12 minutes until the edges are firm but the middle is still soft.
Allow the cookie to cool for 10-15 minutes before enjoying!
Notes
For best results, chill the cookie dough for at least 10-15 minutes in the freezer or longer in the fridge. Using a light-colored baking sheet will prevent the cookie from becoming too brown on the bottom.