Line a baking sheet with parchment paper.
Using a hand mixer or a stand mixer fitted with paddle attachment, beat the butter for 1 minute on high speed until creamy. Switch to medium-high speed and beat in the brown sugar and the granulated sugar until completely creamed and smooth, about 2 minutes. Add the egg and beat until combined, about 1 minute. Scrape down the sides and bottom of the bowl as needed. Add the peanut butter and vanilla extract and beat until combined.
Whisk the flour, baking soda, and salt together in a medium bowl
Slowly and on low speed, add the dry ingredients to the wet ingredients and beat until combined. The dough will be very creamy and soft. Cover and chill the cookie dough in the refrigerator for at least 1 hour
If using any add-ins, add them now, mix in well.
Preheat the oven to 350º while the cookie dough is chilling
Using a 2 Tablespoon cookie scoop, scoop dough and place it on the prepared cookie sheet (do not roll into balls) press gently with a fork if desired, but don't flatten. Reshape with your hands if necessary after pressing with the fork.
Sprinkle the cookies with sea salt (if using) and bake for 11 minutes. Add extra time (2-3 minutes) if you want a crisp cookie.
Let cool on the baking sheet for 10 minutes before moving to the cooling rack.