PREP: Preheat the oven to 350 degrees F. Rub shortening into each cavity of a miniature muffin pan. Dust with cocoa powder, shaking off and discarding any extra. This step is very important so that the brownies come out nicely. I like to use a plastic sandwich bag to keep my hands clean (place the bag on your hand like a glove, dip in shortening, coat muffin tin cavities, repeat). Muffin liners don't work well with this recipe.
BROWNIES: Prepare the brownie mix according to package directions. Fill each muffin cavity 2/3 full. Bake for 17-21 minutes, being careful to not over-bake. They should look set and not wet on top.
BROWNIES, CONT.: Remove from the oven and immediately gently press the back of a 1/2 teaspoon measuring spoon into the center of each brownie. Allow to cool slightly (about 5 minutes) and then gently twist the brownie and pull out. Allow to completely cool on a wire rack.
PEANUT BUTTER FILLING: While the brownies cool, beat the peanut butter and butter together until creamy. Add in the powdered sugar and vanilla extract (Add a pinch of salt if your peanut butter is unsalted). Set in the fridge to firm up while the brownies finish cooling.
PEANUT BUTTER FILLING, CONT.: Fill the completely cooled brownie cups evenly with the peanut butter filling. Use a knife to smooth the top so it is even.
CHOCOLATEGANACHE: I like to separate the ganache into two parts so the chocolate isn't hardening while dipping. Add 2 tablespoons heavy whipping cream, 1/3 cup milk chocolate chips, and 1 teaspoon light corn syrup to a microwave safe bowl. Microwave in bursts of 15 seconds, stirring in between each burst for 15 seconds, until melted and smooth. Dip brownies into the ganache and shake off the extra. Once you've used all that chocolate ganache, repeat with the remaining 2 tablespoons heavy cream, 1/3 cup milk chocolate chips, and last teaspoon corn syrup. Dip until all brownie bites have been topped with chocolate ganache.
STORAGE: Allow to stand at room temperature to completely set up. These bites are best enjoyed within 1 or 2 days of making. Refrigerate leftovers in an airtight container, and allow time for the brownie bites to stand at room temperature (about 30 minutes or so) before eating so the brownies aren't too hard.
Note 1: We love milk chocolate, but if you're concerned about sweetness, use semi-sweet chocolate chips instead. I recommend a good, high-quality chocolate chip for best flavor and best melt.