Mix the flours, yeast, sugar and salt in a bowl.
Gradually whisk in the warm milk until smooth, then beat in the egg to form a thick pouring batter (thicker than crêpes, thinner than drop scones).
Cover and leave somewhere warm for 45–60 minutes until slightly bubbly.
Poffertjes panHeat a poffertjes pan (or frying pan) over medium-low heat and butter generously. Fill each hollow about three-quarters full.Heavy frying pan or Crepe makerHeat a heavy frying pan or electric crêpe maker on medium-low for several minutes. The heat should be gentle, not hot — butter should foam quietly, not sizzle hard.Instead of greasing the whole pan, place small blobs of butter spaced across the surface and allow each to melt into a small puddle.Using a squeeze bottle, piping bag or teaspoon, drop small rounds of batter into each butter puddle, about 4–5 cm wide, keeping them slightly thick rather than letting them spread thin. Cook for about 1 minute until bubbles form and the base is golden, then turn using a skewer or fork and cook the second side for another minute.
Serve immediately piled high with melted butter and a heavy dusting of icing sugar.