Cream the butter, peanut butter, brown sugar, and granulated sugar together.
Mix in the egg yolk and vanilla extract to combine.
Fold in the flour, baking soda, salt, and chopped peanut (if using) just until the flour mixture disappears.
Using a small cookie scoop, measure out 10 equally-sized cookie dough balls and place them on a parchment lined baking sheet. Flatten them slightly using your fingers and place then baking sheet in the freezer for a least an hour. DO NOT skip this step otherwise the cookies will spread too much and we do not want that.
After the cookies have been in the freezer for 55-60 minutes start preheating the oven to 350F. Line another baking sheet with parchment paper or silpat and set aside.
Coat the cookies in granulated sugar and place them on the new parchment lined baking sheet. Make sure to place the cookies at least 2 inches apart because they will spread quite a lot.
Bake for 12-15 minutes or until the edges are set and the tops look like they're just about to be set. You do not want to over bake the cookies because they will lose that perfectly soft attribute.
Let the cookies cool down for 15-20 minutes on the baking sheet before eating!