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Line an 8-inch or 9-inch (20.32 or 22.86 centimeters) square baking pan with wax paper or parchment paper going each direction so that the bottom and all four sides are covered. Leave about 2 inches (5.08 centimeters) of wax paper hanging over the edges. This way, you can use them like handles and pull the finished fudge out. You can also do this using tin foil instead. Stir the ingredients often to help the chips melt evenly. Keep stirring until everything is melted, smooth, and mixed together. There should be no lumps, clumps, streaks, or swirls. If you do not like peanut butter, you can use another type of chip, such as mint chocolate chip, red velvet chocolate chip, etc. The vanilla will help add a more complex flavor to your fudge. The salt will help cut the sweetness as well as bring out the chocolate and peanut butter flavors. Use a spatula to help guide the mixture across the bottom of the pan. Once everything is in the pan, place it into the fridge and wait until it becomes firm. The fudge must be firm before you cut it, or you'll end up with an oozy mess. Once the fudge has hardened up, use the wax paper handles to lift it out of the pan. Peel the wax paper away from the edges. If the fudge is too hard to cut, let it sit on the kitchen counter for 5 minutes. This will allow it to soften a little. Make sure that you are using a sharp knife to do this. If there are any leftovers, cover them up and store them in the fridge. The fudge should last about 1 week.

Summary:
Prepare your baking pan. Heat the condensed milk, chocolate chips, and peanut butter chips in a saucepan over low heat until melted. Remove the saucepan from heat and stir in the vanilla extract and salt. Pour the mixture into the lined pan and refrigerate for at least 2 hours. Remove the fudge from the pan. Cut the fudge into 1-inch (2.54 centimeters) pieces. Serve the fudge on a plate.