Preheat oven to 375.
Mix butter, peanut butter and sugars until very well creamed and mixture gets a couple shades lighter. Add egg, milk and vanilla.
Mix flour, sugar and salt separately, then add to butter mixture. Fold in as many chocolate chips as are desired.
Bake in mini tart pan for 10 minutes or until just browning. Let cool for a few minutes before removing from the pan.