To make the cupcakes, preheat the oven to 180°C/350°F/Gas 4. Line cupcake pans with 24 cupcake liners and set them aside.
Using an electric handheld or stand mixer, place the cake mix, butter, milk, and eggs into a large bowl and mix until combined and smooth.
Stir in the Jimmies until well distributed.
Divide the batter between five bowls and color one each red, yellow, pink, green, and blue. Remember you can always add more coloring, but not take it away! Stir in the food coloring until well combined.
Using a spoonful of each colored cupcake batter at a time, place into each cupcake to create a marbled effect, until the cupcake liners are about 3/4 full.
Bake for 20 minutes or until an inserted toothpick comes out clean.
Leave to cool in the pan for about 5 minutes before transferring to a wire rack to cool completely before decorating.
To make the frosting, use an electric stand or handheld mixer and add the butter, powdered sugar, cream, and vanilla to the bowl.
Beat on medium speed until creamy and smooth and stiff peaks form.
Scoop the frosting into the large piping bag fitted with an open star tip.
Pour the rainbow unicorn sprinkles into a bowl.
Pipe a thick ring of frosting on top of each cooked and cooled cupcake.
Dip the cupcakes into the sprinkles to coat the ring of frosting.
Pipe a small swirl of frosting on top of the sprinkle layer.
Place the unicorn horn and ears icing decorations into the small frosting swirl. Enjoy!
-