Stir yeast into 100 ml of lukewarm milk and let it rise for about 10 minutes. Beat egg yolk and remaining lukewarm milk until frothy. Stir in flour, lemon zest, salt and yeast mixture. Cover and leave to rise in a warm place for about 45 minutes.
Then beat the egg white until stiff and fold in. Grease the depressions of a Poffertje pan. Fill each to the brim with dough and bake until light brown. Turn with a fork and bake from the other side until golden brown.
Results in about 30 pieces. Turn the poffertjes while still warm in the sugar or dust with icing sugar. Serve with syrup or blueberry puree.