Use an 8cm round cookie cutter to cut as many circles as you can out of the large red velvet cake. You need six circles to make three mini cakes.
Use a blob of buttercream to stick one cake round to a serving plate. Spread over a dollop more then place another cake round on top, upside down so that the top is perfectly flat.
Carefully spread a thin layer of buttercream over the whole cake, it is quite soft so be gentle; I found the easiest way to do it was to hold the cake steady by placing two fingers on the top while I iced the sides, then gently spread buttercream over the top.
It will be very full of crumbs, this is fine and exactly why crumb coats are important! Just make sure you don't get any crumbs back in the bowl of buttercream. Repeat these steps to make two more mini cakes then place them all in the freezer for about 15 minutes or so until firm.
Spread another, thick layer of buttercream over each cake, using a dough scraper or pallet knife to get it smooth and even. It should be easier to spread this time as the cakes are firm. Once the cakes are smooth, return them to the freezer for another 15 minutes or so. (Or the fridge if you aren't yet ready to assemble the cakes.)
To assemble the hearts, knead a bit of black fondant into the red fondant to darken it. Break off a piece (about 125g worth) and roll into a ball. Sprinkle a work surface lightly with icing sugar. Roll out the fondant into a rough rectangle about 5mm thick.
Remove the liner from one of the cupcakes and place it in the middle of the rectangle, fold the two long ends over the cupcake to make a parcel.
Trim off some of the excess fondant from one end of the parcel then fold it up. Use your hands to mould it into a sort of heart shape, don't worry too much about any creases or folds, they look great once the hearts have been painted with piping gel.
Trim a bit of the excess fondant off the top and use your fingers to form three arteries.
Repeat with the remaining cupcakes, you can re-use the fondant trimmings. Once the hearts are formed, use a soft paintbrush to brush them all over with the coloured piping gel, I brushed it on quite thickly so it looks like congealed blood. (Don't paint them until you are ready to assemble the cakes. The unpainted hearts can be stored in an airtight container for up to a couple of days but once they are painted they will start to soften a bit.)
Remove the cakes from the fridge or freezer and drizzle some of the ganache over one side of each cake (if it has thickened too much you can give it a very short blast in the microwave to melt it a bit). Place a heart on top of each cake and serve.
If the hearts don't want to sit in place, use a toothpick to pin them to the cakes.