For the sabayon, whisk the egg yolks, wine, sugar and lemon juice in a medium bowl for one minute. Place the bowl over a pan of barely-simmering water and whisk constantly for 7-8 minutes until the sabayon is light and fluffy. Set the bowl over a larger bowl filled with ice and continue whisking to chill the sauce. Cover the sauce with clingfilm and refrigerate overnight.
For the shortbread, place the egg yolks and sugar in the bowl of a food mixer fitted with the paddle attachment. Mix on a high speed for four minutes until the mixture is very pale and doubled in volume. Reduce the speed to minimum, add the flour, baking powder and salt and mix for two minutes. Add the butter, mix for one more minute then scrape the dough onto a baking tray lined with greaseproof paper. Cover the tray with clingfilm and place in the fridge for 35 minutes for the dough to stiffen.
Preheat the oven to 160C/310F/Gas 2½. Roll the dough between two sheets of greaseproof paper to a thickness of 3mm/⅛in. Place the dough on a baking tray and transfer to the freezer for 15 minutes. Remove the top layer of greaseproof paper, cut the dough into four rectangles 5cm x 15cm/2in x 6in and place on a greased baking tray. Bake for six minutes until golden-brown then remove from the oven and trim the edges of the shortbread to make a neat finish. Set aside to cool.
Meanwhile, place the plums, sugar, plum liqueur and lemon juice in a bowl and set aside to macerate for one hour. Cut eight of the plum halves into quarters and set aside. Place the remaining plum halves and the macerating liquid into a pan and bring to a boil. Remove the pan from the heat, add the quartered plums and allow cool to room temperature.
To serve, place the plum halves together to reform into whole plums. Place a shortbread biscuit on each plate and top with three plums. Spoon the quartered plums around and top with a spoonful of the sabayon. Dust the surface with icing sugar twice and gently caramelise with a blow torch.