Preheat the oven to 350°F and line baking sheets with parchment paper.
In a large bowl, cream together the softened butter and brown sugar until smooth.
Add the egg, vanilla extract, and peanut butter, and mix until fully combined.
Add the flour and baking soda, mixing until just incorporated. Do not overmix.
Scoop 1–2 tablespoons of dough and roll into balls.
Roll the dough balls in granulated sugar if desired.
Place the dough balls about 1 inch apart on the prepared baking sheets.
Use a fork to gently press each cookie in a criss-cross pattern.
Bake for 7–8 minutes, until lightly set.
Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
