Preheat oven to 350 degrees F and grease a mini muffin pan.
With a mixer, cream together butter, peanut butter and sugars. Beat in egg.
Add flour, baking soda and salt and beat until combined.
Drop teaspoons of dough into mini muffin pans (or you can use regular pans and just make them twice as big!).
Bake for 9-10 minutes until the center is set. Remove from the oven and use a small, round object to make an indent in the center while still warm (I used the back of a 1 tbsp cookie scoop, and it worked wonderfully!).
Let cool for 20-30 minutes before removing from pan.
In a medium pot, combine chocolate chips and condensed milk over medium-low heat and stir until melted and smooth. Fill cookie shells and set aside to set for about 20-30 minutes.
These store perfectly at room temperature for a couple days, or in the refrigerator or freezer!