Make the Cupcakes:
Preheat the Oven:
Preheat your oven to 350°F (175°C). Line two cupcake pans with 24 cupcake liners.
Prepare the Batter:
In a medium-sized mixing bowl, combine the chocolate fudge cake mix, sour cream, milk, vegetable oil, eggs, and vanilla extract. Beat the mixture on medium-high speed until all the ingredients are well blended and the batter is smooth.
Fill the Cupcake Liners:
Divide the batter evenly among the 24 cupcake liners, filling each one about 2/3 full.
Bake the Cupcakes:
Bake the cupcakes in the preheated oven for 14 to 16 minutes, or until the cupcakes are set. To test for doneness, insert a toothpick into the center of a cupcake; if it comes out clean, the cupcakes are ready. Allow the cupcakes to cool completely on a wire rack after removing them from the oven.
Make the Frosting & Assemble:
Prepare the Frosting:
Using an electric mixer, beat the room-temperature butter on medium-high speed for about 5 minutes, until the butter is light and almost white in color. Be sure to scrape down the sides of the bowl occasionally.
Mix the Cocoa and Brownie Powder:
In a separate bowl, sift together the cocoa powder and powdered brownie mix. Slowly add this mixture to the beaten butter, mixing until well combined.
Add the Powdered Sugar:
Gradually add the powdered sugar to the frosting mixture, beating on medium speed. The frosting will become very thick. To maintain a smooth consistency, alternate adding the milk and vanilla extract with the powdered sugar. Continue beating the frosting on high speed for 2-3 minutes until fluffy.
Finish the Frosting:
Add the light corn syrup to the frosting and beat until well incorporated, creating a smooth and rich frosting.
Assemble the Cupcakes:
Pipe the frosting onto the cooled cupcakes using a large open star tip. Garnish each cupcake with 3 small pieces of brownie and drizzle with hot fudge sauce.