Preheat oven to 350℉. Grease a baking sheet with butter and set aside.
With a whisk or a good wooden spoon (or with a mixer) combine the 1 cup smooth peanut butter, 1 cup brown sugar, and ½ cup granulated sugar until well combined. Add 1 teaspoon maple syrup if using, 1 egg, 1 teaspoon baking soda, and ½ teaspoon fine sea salt and mix well to form a dough. The dough will be wet.
With a teaspoon, scoop out balls of dough and roll into balls, then press lightly in a cross-hatch pattern with a fork dipped in water. Bake for 10-15 minutes, until lightly browned. Cool on a baking sheet for two minutes.
Notes
What size cookie? In the process shots above, you can see that I made larger cookies, which does tend to allow them to run into each other a bit. This doesn't bother me, but you may wish to spread them out further or to make slightly smaller cookies. It's up to you. Larger cookies will take a tad longer to bake.
How long to bake? Again, this is personal preference. For crunchier cookies, bake for about 15 minutes. For softer, more chewy cookies, bake for less time.
The dough will be wet. Note that the dough will be quite loose and wet, but you will still be able to form the cookies. If you want, pop the bowl in the fridge for 10 minutes or so to help it firm up.
Sprinkle with sugar. For a little extra sweetness, dust the tops of the cookies with coarse or granulated sugar before baking.
Make vegan. This is a dairy-free recipe that's easy to convert to being vegan. Swap the egg for 1 teaspoon of cornstarch.