Open-freezing means you freeze a dish just until it is firm, without wrapping it. Then, when you do wrap it, it won't get squashed.
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 in wedges.