Preheat oven to 350 degrees.
Place parchment paper onto a cookie sheet and set aside.
Place stick of butter into a small pot over medium heat. Once butter starts to melt, start swirling the pan. The butter will start to foam. Keep swirling the pan until you see the colour start to change and the butter starts to smell nutty. Remove from the heat.
To a stand mixer, add butter, miso and sugar. Blend together until sugar has fully dissolved. Scrape down sides as needed. Add egg, peanut butter and vanilla. Process again until fully incorporated.
Add flour, baking powder, soda and salt. Place a kitchen towel over the machine to prevent a flour cloud and process until flour mixture is thoroughly combined. If you are adding chocolate chips, add them in and process once more to combine.
Use an ice cream scoop to measure out 5-6 cookies onto a cookie sheet, making sure to leave 2" between them to allow for spreading.
Bake for 12 minutes. Firmly bang the cookie sheet to help the cookies fall. You can bake them again for 1-2 minutes or allow them to cool on the cookie sheet before transferring them to a wire rack.
Sprinkle flaky sea salt if desired and enjoy!