Get started: Heat oven to 160C/140C fan/gas 3. Line a 12-hole muffin tin with paper cases. Gently melt the butter, chocolate, sugar and 100ml hot water together in a large saucepan, stirring occasionally. Set aside to cool a little while you weigh the other ingredients.
Make your cakes: Stir the eggs and vanilla into the chocolate mixture. Put the flour in a large mixing bowl, and stir in the chocolate mixture until smooth. Spoon into the cases until just over three-quarters full. Bake on a low shelf in the oven for 20-22 mins. Leave to cool.
Ice the tops: To make the icing, melt the chocolate in a heatproof bowl over a pan of barely simmering water. Once melted, turn off the heat, stir in the double cream, sift in the icing sugar and mix well. When spreadable, top each cake with some icing.
Have fun decorating: Position a milk chocolate button on top of each cake, then 2 white chocolate buttons above it. Use a little icing as glue to stick a red Smartie onto the milk chocolate button for a nose. Then use your icing pens to draw black dots on the white buttons for eyes. Stick 2 pretzel top halves into the top of each cake for antlers, and stick the bottom half of a pretzel under the Smartie for a mouth. These cakes will keep in a sealed container for up to 3 days, but we doubt they’ll last that long!