Preheat oven to 325 degrees.
Sift together flour, baking soda and salt.
Cream butter, peanut butter and white and brown sugars. Mix in egg and ½ teaspoon vanilla. Stir the flour mixture into creamed mixture. Shape the dough into 48 balls, 1 inch each. Place each ball in one compartment of a mini muffin tin sprayed with cooking spray.
For filling, put chocolate chips in a double boiler over simmering water. Stir in sweetened condensed milk and vanilla, mix well.
Bake for 14 to 16 minutes or until lightly browned. Remove from oven and immediately make wells in the center of each using a small spoon, melon baller, or your thumb.. Cool in pan for 5 minutes. Then carefully remove to wire racks. Fill each shell with the chocolate mixture.