In a mixing bowl, cream butter, peanut butter and sugars; add egg and vanilla; stir together flour, baking soda and salt; add to creamed mixture. Mix well. Chill for 1 hour. Shape into 48 balls. Place in lightly greased mini-muffin tin. Bake at 325 F degrees for 14-16 minutes or until lightly browned. Remove from oven and immediately make wells in the center of each by lightly pressing with a melon baller. Cool in pans for 5 minutes. Then carefully move to wire racks. For filling: melt chocolate chips in a double boiler over simmering water. Stir in milk and vanilla; mix well. Using a small pitcher or pastry bag, fill each shell with filling. Sprinkle with peanuts. (Leftover filling can be stored in the refrigerator and served warm over ice cream)