Want to skip the skip of making cupcakes from scratch? No problem, buy plain ones from the store or make them from a box mix, skip to step 5.
Preheat oven to 350°F. Line cupcake pans with liners (around 30).
Make the cupcakes: whisk the flour, sugar, cocoa powder, baking soda, and salt in a large mixing bowl. Whisk the hot water, oil, vinegar, coffee (if using), and vanilla in a large measuring cup. Whisk, then stir, the liquid into the dry mixture.
Divide evenly among cupcake liners (about 2/3 full) and bake for about 15 minutes, until a toothpick comes out clean. Cool completely before filling and frosting.
While the cupcakes are baking, make the frosting. The frosting requires cooling/chilling time. Melt the butter in a medium saucepan over medium low heat. Stir in the sugar and cocoa. It will make a thick paste. Whisk together the heavy whipping cream, sour cream, vanilla, and coffee (if using) in a large measuring cup. Slowly stir the cream mixture into the saucepan, whisking as needed, until smooth. Cook until the mixture is thick and hot, 4-8 minutes (just until it starts to bubble), then cool for 2-3 hours. I like to cool it until the bowl is easy to touch, then chill it until it’s thick like frosting, stirring occasionally. Chilling will help it set faster, especially if it’s warm in your house.
Make the filling: beat peanut butter and sugar with a hand mixer or stand mixer fitted with the whisk attachment until smooth. Add in vanilla, then slowly add whipping cream and turn the speed to high. Beat until stiff peaks form. Chill until ready to use.
To assemble cupcakes: fill each cupcake with some of the peanut butter filling. You can either use a knife to cut out some of the cupcake and spoon or pipe it in, or you can use a Wilton Cupcake Filling Tip (Bismarck) and a pastry bag to fill by just pushing the tip into the cupcake and moving it slightly as you press the bag to fill the cupcake. Frost with the frosting and sprinkle with chopped peanut butter cups.
Store in refrigerator up to 3 days. Cupcakes can be made and frozen ahead of time for up to one month. Filling should be used within 24 hours (store in refrigerator) and frosting can be made up to 2 days ahead and stored in the refrigerator (it might harden completely, let come to room temperature and stir to loosen).