Preheat the oven to 325 degrees F. Spray the mini muffin pan with the cooking spray. Mix the butter, peanut butter, sugar and brown sugar together in one bowl. Add an egg and vanilla extract while mixing. Stir in the flour, baking soda and salt. Form little balls of dough and place each one in one compartment of the mini muffin pan. Bake the cups for 14 minutes and as you take it out from the oven immediately press every cookie in the middle to make enough space for an upcoming fudge. While the cookies are cooling pour the chocolate chips and condensed milk in the bowl and put it in the microwave oven to melt. Stir it well until smooth. Add the vanilla extract. Fill the ziplock bag and snip one corner of the bag off. Squeeze the fudge into every cup to fill it up.