Preheat the oven to 350°F and line a baking sheet with parchment paper. Set aside. In a stand mixer fit with the paddle attachment, beat the softened butter, peanut butter, white sugar and brown sugar together until light and fluffy.
Scrape down the bowl and add the egg, egg yolk and vanilla extract. Mix well. In a separate bowl sift together the flour, baking soda, and salt. Add the dry ingredients to the wet ingredients and mix until just combined.
Pour some extra granulated sugar into a small bowl. Using a cookie scoop, scoop the dough out into 1 inch balls, rolling them in the granulated sugar first before dropping them 2 inches apart on the prepared cookie sheet. Bake at 350°F for 12 minutes or until the cookies have puffed up and are set on the edges, but are still gooey in the middle. Allow to cool completley before eating!