Mix the yeast with 3 to 4 tablespoons of the milk and let stand for a few minutes until it starts to foam.
Mix yeast with the rest of the milk, 50g butter, salt, half of the caster sugar, egg, lemon zest and flour in a bowl. Work all ingredients into smooth dough, using the kneading attachment of an electric hand-held mixer. Cover with a damp tea towel and let stand for 45 minutes in a warm place.
Place the dough on a floured work surface and form a roll of about 25cm. Cut it into 12 equal portions and shape into small dumplings with floured hands. Cover and let stand for 15 minutes in a warm place.
Slowly heat additional milk in a large, wide saucepan with a tight-fitting lid. The milk should fill the pan to at least 2cm. Add 75g butter and the rest of the caster sugar and the vanilla.
Place the dumplings in the milk. Cover the pan immediately. Cook at medium heat for 20-30 minutes. Opening the pan during the cooking process is an absolute no-no, or your dumplings will collapse.
Open the pan only when you hear a sizzling sound, which indicates that the dumplings have a nice brown crust at the bottom while light and shiny on the top – this is exactly the way they should be. Remove the dumplings with a pancake turner one by one and place them bottom up on a large serving platter. Serve the dumplings warm with fruit compote or chilled vanilla custard.
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