Preheat oven to 350°F and line a baking sheet with parchment paper.

In the bowl of an electric mixer, beat peanut butter, coconut sugar, and butter for 1 minute, until smooth. Beat in egg and salt. Stir in peanuts.

Using a mini ice-cream scoop or tablespoon, scoop dough into 1 3/4-inch-diameter balls. Arrange dough balls 2 inches apart on prepared baking sheet. Using back of fork, flatten dough balls and form crosshatch design on tops. Bake cookies until edges barely begin to turn golden brown, about 9 to 12 minutes. Sprinkle with flaky sea salt, if desired. Let cool slightly on baking sheet, then transfer to a wire rack and cool completely. Repeat with remaining dough balls.