Preheat oven to 350℉. Line baking sheet with parchment paper.
In a small glass bowl, microwave unsalted butter (½ cup, i.e., 1 stick) until melted. Pour into a large mixing bowl.
Add in granulated sugar (¼ cup), light brown sugar (½ cup), and peanut butter (½ cup) and mix for 10 seconds.
Add egg (1 large) and vanilla extract (1 ½ teaspoons) and mix on medium speed for 20 seconds.
Turn mixer to low speed, and add baking soda (½ teaspoon) and salt (¼ teaspoon). Mix for 10 seconds.
Half cup at a time, add all-purpose flour (1 ½ cups) and mix until flour is just incorporated (about 10-20 seconds). Do not overmix.
Scoop dough into (3 tablespoon sized) balls. Lightly flatten each ball before placing on cookie sheet.
Bake at 350℉ for 8 minutes. They will look underdone, but they will finish baking on the hot cookie sheet after you take it out of the oven. (Of course, because ovens bake differently use your best judgement to keep them in a minute longer if they look severely underbaked.)
After removing from oven, allow the cookies to cool on the hot baking sheet for 10 minutes before removing them from cookie sheet. If dipping in chocolate, allow to cool completely on cooling rack.
For chocolate dip, melt chocolate in microwave at 50% power in 30-second increments until melted. Be sure to stir between each increment.
Immediately dip cookies in chocolate and place on wax paper or parchment to dry. If needed, pop the cookies in the refrigerator to help harden.