Divide the blackberries – three-quarters of the punnet into a bowl, the remaining quarter on some kitchen paper to absorb juice.
Add the tablespoon of sugar to the blackberries in the bowl and gently crush the fruit as you stir to mix.
Fill a pan halfway with water and put it on high heat until steaming and beginning to simmer. Meanwhile add the egg yolks, 75g sugar and the vanilla to a heat-proof bowl and stir until combined.
Once the water is steaming turn off the heat and put the bowl over the pan. Whisk the eggs with electric beaters until thick and pale. The heat from the steam speeds up this thickening process.
Pour in the wine and taste – and if you want add more! – before whisking the mixture again, this time for 10 minutes or so. The custard should show defined marks from the whisk and, when you gently pull the beaters out of the mixture, a prominent ribbon of custard should fall back and remain defined for 5-7 seconds.
At this stage the eggs are ready. Divide the crushed blackberries between four glasses and spoon over dollops of custard. Decorate with the rest of the blackberries and grated chocolate.