Next, whisk in the egg, egg yolk, and vanilla extract until well combined, scraping the sides of the bowl as needed. Gradually add the flour mixture to the wet ingredients, mixing until just combined. Be careful not to over-mix; a few streaks of flour are okay. Once the peanut butter cookie dough is ready, refrigerate it for 15 minutes, and preheat your oven to 350°F (175°C).