Preheat the oven to 350 degrees. In a large bowl, mix together the sugar, flour, cocoa powder, baking powder, baking soda and salt. In a separate bowl, mix together all of your wet ingredients (egg, oil and vanilla only). Add the wet into the flour mixture and slowly add your hot coffee and mix until incorporated.
Mix together the creamy peanut butter + the confectioners' sugar until well combined. Using your hands, roll the peanut butter mixture into small sized balls (you just need one for each jar).
Scoop out about 2-3 tablespoons of the cake batter (I used a cookie scoop to release the batter easier into the jars) and fill the bottom portion of the jar. You want enough to create a nice bottom layer of cake. Next, place one peanut butter ball into each jar and cover with the remaining cupcake batter in each jar.
Place the jars at least 2 inches apart on a baking sheet and bake for about 18-20 minutes or until the cake bounces back when touched.
(Have your Reese's Peanut Butter cups unwrapped and ready to go) As soon as you remove the jars from the oven, place the Reese's Peanut Butter cups on top (in each jar) and allow to melt into each other. Smooth together with a spoon, allow to cool slightly and enjoy!