Preheat oven to 350°F. Line baking sheet with parchment or silicone baking mat.
In a large bowl, combine butter, brown sugar, and white sugar. Cream together with a hand mixer. Add egg + egg yolk along with vanilla, mix until just combined.
In a separate bowl, mix flour, baking soda, and salt. Add to wet ingredients and mix by hand. Fold in chocolate. Doing this by hand prevents over-mixing.
Portion into 9-12 dough balls and bake for 10-12 minutes. They'll still be soft and will look a little underbaked, but that means you did it right.
