Preheat your oven to 350°F (175°C). In a large mixing bowl, whisk together the all-purpose flour, cocoa powder, baking soda, baking powder, granulated sugar, brown sugar, and salt until well combined.

In a separate bowl, mix the oil, eggs, buttermilk, and vanilla extract. Slowly pour the wet mixture into the dry ingredients, stirring gently. Gradually add steaming hot water to create a smooth batter.

Line a muffin pan with cupcake liners and fill each liner about two-thirds full with the batter. Bake for about 16-20 minutes, checking with a toothpick.

Let the cupcakes cool in the pan for a few minutes before transferring them to a wire rack to cool completely.

In a medium bowl, beat together the peanut butter, marshmallow fluff, milk, and ground cinnamon until smooth. Gradually mix in sifted powdered sugar until pipeable.

Cut a small circle out of the center of each cooled cupcake and fill with the peanut butter mixture using a piping bag.

Beat the unsalted butter until fluffy. Gradually add peanut butter, then sifted powdered sugar, mixing until combined.

Add heavy whipping cream and mix until light and fluffy. Chill for about 15 minutes before piping onto cupcakes.

Top each frosted cupcake with half of a Reese's Peanut Butter Cup if desired.