Preheat oven to 325 °F (163 °C).

In a large bowl, cream together butter, peanut butter, white sugar, and brown sugar until light and fluffy. Mix in the egg and vanilla until smooth.

Stir in flour, baking soda, and salt until just combined. Dough will be soft.

Roll dough into 1‑inch balls and place each into an ungreased mini muffin tin. Use a measuring spoon or your thumb to press an indent in the center and form cookie cup walls.

Bake for 12–14 minutes, or until edges are lightly golden and centers puff.

Remove from oven and immediately press down centers again gently to deepen cup shape. Cool in pan for 5 minutes, then remove to wire rack to cool completely.

To make the fudge filling: melt chocolate chips with sweetened condensed milk in a double boiler or microwave in short intervals, stirring until smooth. Stir in vanilla.

Spoon or pipe filling into cooled cookie cups. Top with sprinkles or chopped nuts if desired.

Store in an airtight container at room temperature for up to 1 day, or refrigerate for up to 5 days.