Preheat oven to 350°F and line a small baking sheet with parchment paper.
Use a fork and a small bowl and mash together the softened butter, shortening, brown sugar, and granulated sugar until totally combined. Once combined, cream with the fork another 30-60 seconds.
Add the peanut butter and vanilla extract and mix in completely. Add the whisked egg and use the fork to incorporate until the egg is just combined.
Sprinkle the flour, baking powder, and salt over the dough and mix in until just combined. The flour will incorporate easily and the dough will be soft and a little sticky.
Use your hands to shape the dough into a ball. Don't leave any dough behind in the bowl. Flatten the cookie dough ball so it's about a 1" high disk (about 3" diameter).
Bake cookie about 18-20 minutes until desired done-ness. The edges will be golden brown and forming cracks, the center will be just set, but still gooey and soft.
Allow the cookie to cool at least 15 minutes before moving it to to a plate (or my preferred, a cooling rack). Wait another 15 minutes for a sturdier cookie, or dig on in. Enjoy!