Melt the butter in a small bowl in the microwave, using 8s increments
Add the white and brown sugar, and whisk for 1-2 minutes
Add half of an egg yolk and whisk again for 30-60 seconds. The mixture should look very smooth at this point
Sift in the flour, baking soda and salt, and use a small rubber spatula or spoon to very gently fold until the flour is almost combined (but not fully)
Add the chopped chocolate and continue to gently fold only until no more streaks of flour are visible. You should not overmix after adding the dry ingredients and chocolate!
The batter will be a bit runny at this point - don't worry and chuck into the freezer for 10-15 minutes
In the meanwhile, pre-heat the oven to 375F and line a small baking sheet with parchment paper
Once the dough is chilled, gently form it into a ball, and place onto the baking sheet
Bake for just 10-12 minutes until the cookie is firm around the edges but a bit soft in the middle
Remove from the oven and let cool for 10-15 minutes before eating!
