ready in 2 h. 20 min.
For the dough, mix the flours together in a bowl and make a well in the centre. Add the yeast (if using fresh yeast, break it into small pieces) and 1 tbsp sugar and stir together with a little milk and a little of the flour to make a dough starter. Dust the top with flour, cover and leave in a warm place for around 15 minutes for the yeast to start working.
Add the butter, a pinch of salt, the eggs, vanilla sugar, remaining sugar and lemon zest to the bowl and knead to form a smooth dough, either in a food processor or using the dough hooks on a hand mixer. Add enough milk while kneading to create a dough that is soft but does not stick to the sides of the bowl. Cover the dough and leave for around 30 minutes to double in size.
Put a roasting tin on the hob and melt the butter over a low heat. Divide the dough into roughly 20 pieces of equal size, form into balls and toss in the butter. Arrange in the tin, not too close together, and leave for a further 15 minutes to rise.
Meanwhile, heat the oven to 180°C (160°C in a fan oven), 350°F, gas 4.
Bake the dumplings in the oven for around 30 minutes until golden brown. Serve lukewarm or cold.