Preheat the oven to 180°C/350°F and line two cookie sheets or sheet pans with parchment paper.
Combine the peanut butter, sugar, egg and salt/vanilla (if using) in a mixing bowl and mix with electric beaters or a whisk until well combined.
Form small balls of dough by rolling in between your hands then place on the prepared sheets, a few centimetres apart. Create a pattern by pressing down on the cookies with a fork. Don't flatten too much as they will spread while they bake.
Place the cookie sheets in the oven and allow to bake for 12-15 minutes or until the cookies are golden brown and crisp around the edges.