Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat to prevent sticking and help the cookies bake evenly.

In a medium bowl, stir together the peanut butter and sugar with a sturdy spoon or spatula. At first the mixture will look dry and crumbly, then it will smooth out into a thick paste. Continue stirring until no streaks of sugar remain.

Crack the egg into the bowl. Add a pinch of fine sea salt if using unsalted peanut butter and the vanilla extract if using. Mix until the dough is thick, glossy, and uniform. It should be soft but able to hold its shape. If it looks greasy, continue stirring until it comes together.

Use a small cookie scoop or a tablespoon to portion the dough into balls, about 1 to 1 1/2 tablespoons each. Roll each portion gently between your palms to make smooth balls and place them on the prepared baking sheet, spacing about 2 inches apart.

Lightly press each dough ball with a fork in one direction, then again in the opposite direction to form a criss-cross pattern. If the fork sticks to the dough, dip the tines in a little sugar before pressing.

Bake the cookies for 9–11 minutes, until the edges look set and the tops are just starting to look dry. The centers will still feel soft; this is what gives a tender, chewy texture.

Remove the baking sheet from the oven and let the cookies cool on the pan for 5–7 minutes so they can firm up. Then transfer them to a wire rack to cool completely or until just warm before serving.

After tasting the cookies, adjust the baking time for your next batch if needed. For softer cookies, bake 1 minute less; for crisper edges, add about 1 minute, keeping in mind that they firm up as they cool.