Preheat oven to 350F. Line a baking sheet with parchment paper.

In a mixing bowl, stir together peanut butter, erythritol, egg, vanilla, baking soda, and salt until completely smooth and uniform, about 1 minute. The dough will be thick and slightly oily.

Scoop 1.5 tablespoons of dough and roll into balls. Place 2 inches apart on the prepared sheet. Press each ball with a fork to make a crisscross pattern, pressing to about 0.5 inch thickness.

Bake until the edges are set and tops look dry with small cracks, 10-12 minutes. The centers will still look slightly underdone - this is correct.

Let cookies cool on the baking sheet for 10 minutes. They will firm up as they cool. Sprinkle with flaky salt if desired, then transfer to a wire rack to cool completely.