Preheat oven to 375 degrees.  Line a baking sheet with parchment and set aside.

1. Melt your butter in a heat-safe mixing bowl.  Note: It doesn't need to be ALL the way.  See photos for an example- I've found the best stage is when about half the butter is liquid, but the rest is super soft.  Like the texture of pudding.  You want it to easily stir completely smooth when you touch it.

2. Stir in both white and brown sugar, peanut butter, then egg yolk and vanilla.

3.  Measure your flour by lightly spooning flour into your measuring cup and leveling with a knife.  Don't scoop your flour with your measuring cup.   Sprinkle flour, baking soda, and baking powder in the bowl and stir until it forms a cohesive dough.

4. Form one single ball of dough and set it back in the mixing bowl.  Sprinkle a couple tablespoons of additional sugar over it and roll it around until it's coated with a layer of sugar.

5. Place dough ball on your parchment-lined baking sheet and gently press down with your hand until it's about 1 inch thick.  If desired, use a skewer to form the traditional criss-cross lines on cookie (see photos in blog post for a visual).

6.  Bake for 13-15 minutes or until edges are golden and top looks set.  It should still jiggle in the center when you pull it out- the center will continue cooking as it sits and be soft when it cools.  Let cool for 15 minutes or so before using a very large spatula to transfer to a cooling rack.