In a small bowl, add flour, salt, baking soda and baking powder and whisk together. Set aside.
Line a small baking sheet with parchment.
In the bowl of your stand mixer with paddle attached, or a large bowl with a hand mixer add your melted butter and brown sugar and beat until smooth and creamy.
Add in your vanilla and peanut butter and mix well. Make sure to scrape the bottom of the bowl if using a stand mixer.
Add in your egg and combine.
Add your dry ingredients and mix on low until just combined and no dry bits remain.
Add your chopped peanut butter chips and stir in with a rubber spatula.
Scoop your cookies onto the lined baking sheet, sprinkle with demerara sugar and cover. Place in the refrigerator to chill for 2-4 hours.
When your cookies have chilled for at least 2 hours, preheat your oven to 350°F.
Line a larger baking sheet with parchment and transfer some of your cookies to the new pan. I baked 6 cookies at a time on a half baking sheet.
Bake your cookies on the center rack of your oven for 14-15 minutes. Cookies will be set but soft, with the edges just starting to brown.
Let cookies sit on baking sheet for about 2 minutes before transferring to a wire cooling rack. Sprinkle with flaky sea salt while still on the cookie sheet.
Let cookies rest for 20-30 minutes for the best texture before eating. Store in an airtight container for up to 5 days. Enjoy!
