Preheat the oven to 350F and grease or line a baking tray with parchment paper.
In a large mixing bowl, mix the melted butter and sugars together until combined.
Add the egg and vanilla and mix until the egg is fully incorporated into the wet mixture.
Add the peanut butter and mix until well combined.
With the mixer on low speed, slowly add the dry ingredients and mix until just combined. Do not overmix the cookie dough.
Using a cookie scoop drop each cookie dough dollop about 2 inches apart. If you don’t have a cookie scoop, roll the cookie dough into a 1-inch ball. Then using the prongs of a fork, lightly flatten the ball and make a criss-cross design on the top. Don’t use the fork if you’re using a cookie scoop.
Bake for 9-10 minutes. Sometimes, if the cookies are looking a little too thick, I use a spatula or flipper to flatten them down a little when they come out of the oven.
Transfer to a cooling rack to finish cooling before transferring them to an airtight container or plastic bag.