Place the candy bars in a food processor and process until chopped. (Or you can use a large, sharp knife to cut the candy bars into small pieces, dipping the knife into hot water and wiping it off after every few cuts.)
Combine the flour, eggs, granulated sugar, brown sugar, shortening, milk, baking powder, vanilla and salt in a large bowl. Beat with an electric mixer set on medium speed, scraping the bowl down once, for 2 minutes. Stir the candy bars into the batter.
Divide the batter evenly among the prepared muffin cups, filling each about half full.
Bake in a preheated 350º oven until a toothpick inserted in the center of a cupcake comes out clean, 15-20 minutes. Cool the cupcakes in the pan for 2 minutes, then transfer to a wire rack to cool completely. These don't need frosting.