Make the cupcakes. In a large bowl, add the cake mix, water, oil and eggs. Stir to combine the batter. Add to lined cupcake pan, and bake in oven according to package instructions. Allow to cool on a wire rack before frosting.
Allow the cupcakes to cool completely before frosting.
Make the frosting. For the frosting, beat the butter until smooth. Alternate adding the powdered sugar, vanilla extract, and small amounts of the heavy cream, until the frosting is well combined and the texture is stiff enough to hold it's shape. I like to do this in the bowl of a stand mixer with a paddle attachment on medium speed to save my hands! Frost the cupcakes using a 1M star tip and piping bag. If you don't have a piping bag, you can use a Ziploc bag with the tip cut off!
Make the firecrackers: Place the red candy melts in a microwave safe bowl. Microwave for 30-second intervals, stirring in between until melted. Drop the marshmallows into the candy melts. Stick a tooth pick into the marshmallow, then swirl it around to get it fully coated.
Use a second toothpick to push the marshmallow off the inserted toothpick and onto a piece of parchment paper. Allow it to rest for a few minutes until the candy shell hardens.
Cut the Pull and Peel Twizzlers into about 3/4 inch pieces. Pull off one strip of the cut Twizzler segment, and press it into the top of the marshmallow to make the firecracker "fuse." You can use a bit of extra candy melts to help it stick if needed.
Assemble: Frost the cooled cupcakes, then gently press a firecracker marshmallow into the center of each cupcake. Sprinkle with sprinkles.