Crumble the cake into a large bowl, pour in a few drizzles of alcohol and then gather the crumbs together to form a ball. Refrigerate for at least half an hour.
Melt the milk or dark chocolate in a small bowl, either by placing it over a saucepan of simmering water or in a microwave (I melted mine in the microwave, but for 20 or 30 seconds at a time as it's relatively easy to 'burn' the chocolate while melting it this way).
Take small walnut-sized pieces of the cake mixture and roll them into a ball. Do this quite quickly as the dough is easier to work with when it's cold.
Using a cocktail stick, fork or something similar (I used a kebab stick!), dip the balls into the chocolate. I found it worked best to dip each ball so that 3/4 of the ball was covered in chocolate, then I quickly turned it over and put it onto a plate so that the chocolate ran down the truffle and covered most of the rest.
Let the chocolate on the truffles set.
Meanwhile, slice the red and green jelly sweets/candies into tiny pieces to vaguely resemble berry and leaf shapes (see notes for more details).
Melt the white chocolate, then drizzle a small teaspoonful over the top so that it looks like custard or sauce.
Before the white chocolate sets, place a few tiny pieces of red candy and two longer pieces of green candy on the top of each truffle.
Notes
By following my 'chocolate dipping instructions' above, you may find that a tiny bit of each truffle isn't covered in chocolate. That's OK, in my opinion! I found that if I tried to dip the whole truffle in chocolate, the chocolate ran off the truffle all over the plate and looked messier in the end. Don't worry too much, though. It isn't a precise art!For my 'berries', I just cut tiny tiny pieces of red jelly candy off a jelly snake and put 2 or 3 on each truffle. For the 'leaves', I cut slightly longer, thinner pieces to roughly look like leaves. Again, this doesn't need to be all that precise, in my humble opinion. You just want to create the effect of holly leaves and berries.