Lightly grease four 12-13cm ovenproof blini pans or ramekins with butter.
Preheat the oven to 180°C/160°C fan/gas 4.
Put the demerara sugar in a small heavy-based saucepan or frying pan. Place over medium heat and leave to melt and caramelise to a deep golden colour, swirling the pan occasionally (do not stir) to get even caramelisation – this will take 5-8 minutes. Add the butter, whisk well and simmer for 1-2 minutes until well combined.
Pour a quarter of the caramel into each blini pan or ramekin then quickly, while the caramel is still hot, place 3 plum halves, cut side down, into the caramel.
Put the cream, milk, vanilla seeds and lemon zest in a small saucepan and gently bring to the boil. Remove from the heat.
In a deep bowl, whisk together the eggs and sugar. Whisk in the flour, then gradually add the hot milk and cream, whisking continuously.
Pour the batter over the plums in each pan or ramekin then place in the oven for around 15 minutes, until golden and cooked through. Remove from the oven, dust with icing sugar and serve immediately with your favourite ice cream.