For the zabaglione cream: Put the egg yolks and sugar into a large bowl. Whisk with an electric hand beater for 2 mins until lighter in colour and falling in thick ribbons. Continue to beat while gradually pouring in the wine, then the marsala. Pour this into a medium, preferably non-stick saucepan. Stir over a lowish heat until it thickens. It takes 5-7 mins, by which time it should thickly coat the back of the spoon and be light and frothy. Pour into a bowl and chill for 45 mins, or better still, overnight.
Meanwhile, slice the panettone and lay it in overlapping slices on the bottom (not up the sides) of a 20cm loose-bottomed, round cake tin (5.5cm deep), pressing the slices down so they completely line the base (you are creating a panettone layer). Drizzle over the 3-4 tbsp of marsala, so each slice gets a little bit, and let it soak in. Tip the drained cherries onto kitchen paper and pat dry (if they are too wet the panettone will go soggy when it thaws), then halve each one to make it easier to cut later.
Whip the cream into soft peaks and fold it into the cold marsala mixture. If it all seems a bit lumpy, give it a quick beat with a wire whisk. Pour the zabaglione cream over the cherries and give it a few swirls so it is slightly undulating on top.
Open-freeze until firm, then cover with cling film and foil, and freeze for up to 1 month.
When ready to use, unwrap and thaw in the fridge for about 2-2½ hrs (it should be quite moussey), then serve, generously dusted with sifted cocoa powder and topped with holly. Serve cut into wedges.