Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a medium bowl, stir together the peanut butter, erythritol, cocoa powder, egg, vanilla extract, baking soda, and salt until a thick, fudgy dough forms. Fold in the sugar-free chocolate chips.
Scoop tablespoon-sized portions of dough and roll into balls. Place them on the prepared baking sheet about 2 inches apart. Use a fork to press a crosshatch pattern into the top of each cookie, gently flattening them.
Bake for 10 to 12 minutes, until the tops look set and have small cracks. The cookies will still feel slightly soft in the center.
Let the cookies cool on the baking sheet for at least 10 minutes to firm up before transferring to a wire rack. They will become fudgy and chewy as they cool.
Serve at room temperature. Store leftovers in an airtight container at room temperature for up to 5 days.