In a large mixing bowl, beat the butter, peanut butter, and sugars until creamy.

Beat in egg and vanilla.

In a small bowl, whisk together flour, baking soda, and salt.

Add flour mixture to the mixing bowl, and beat until combined.

Chill dough for at least 1 hour.

Preheat oven to 325°F.

Scoop dough by small teaspoonfuls into lightly greased mini muffin tins.

Bake for 12-15 minutes, or until edges are golden brown.

Remove from oven, and immediately press the end of a wooden spoon into the center of each cookie to make a well or indentation – do not press all the way to the bottom.

Cool in pan for 5 minutes before removing to wire racks to cool completely.

In a small microwave-safe bowl, combine chocolate chips and sweetened condensed milk.

Heat in the microwave for 30 seconds.

Stir, and heat an additional 10-15 seconds, or until smooth.

Spoon or pipe mixture into the center of each cookie cup.

Allow to set, about 10-15 minutes.