Pre-heat oven to 350° F. Line a baking sheet with parchment paper and set aside.
In a medium-sized bowl, cream together the butter, peanut butter, and both sugars. Mix until smooth. Add the egg yolk and vanilla and mix until combined.
To this, add in the flour, baking soda, and salt. Mix until just combined, being sure not to over mix the dough at this point.
Scoop about 2-2½ tablespoon of dough per cookie and gently roll them into balls. Place on a parchment paper-lined baking sheet, leaving a few inches in between cookies. If you have excess dough, just add them to the existing cookies (they should be fairly large in size).
Bake the cookies for 10-12 minutes, just until the edges start to look lightly golden brown. The tops will seem under-baked, but let the cookies rest on the cookie sheet for 5-10 minutes before transferring to a wire cooling rack to chill completely.
spoon and level your flour- I always recommend spooning and leveling your flour when baking. Scooping the measuring cup right into the flour will actually compact the flour and give much more than the recipe calls for. To spoon and level, use a large spoon to gently scoop the flour into your measuring cup. Once it's full, use a flat edge (like a butter knife) to gently scrape across the top of the measuring cup.
roll your cookies in sugar before baking- this is optional, but rolling your peanut butter cookies in granulated sugar before baking gives them an extra hint of sweetness you won't be able to resist!
let the cookies cool on the pan- this is an important step because the cookies will come out slightly underbaked, but will continue to bake while cooling on the baking sheet. This helps give you that soft and chewy texture.