Poffertjes are one of the best Dutch desserts, and their preparation requires a special pan (poffertjespan) which has small hemispherical indentations. If you don't have one, you can use a regular pancake pan, but they won't be as perfectly round.
Poffertjes batter is thin and contains yeast, which contributes to their fluffy texture. Buckwheat flour gives them a slightly nutty flavour. They are best immediately after cooking, hot and coated in a sugar blanket.
In a small bowl, mix the yeast with a little lukewarm milk and let it activate for 10 minutes.
In a large bowl, mix the all-purpose and buckwheat flour with salt.
Add the remaining milk, egg, and activated yeast to the flour. Whisk until smooth, ensuring no lumps. The batter will be quite thin.
Cover the bowl and let it prove in a warm place for 45 minutes until it doubles in volume and bubbles start to form.
Heat the Poffertjes pan (or a deep mini-pancake pan) well over medium heat and grease with melted butter or oil.
Pour the batter into each indentation (ideally using a squeeze bottle or piping bag) – do not let it overflow.
When bubbles start to appear on the surface and the bottom side is cooked (about 1-2 minutes), flip the pancakes with a skewer or small fork.
Cook the other side for about 1 minute until golden brown.
Stack the hot Poffertjes on a plate. Drizzle with melted butter and generously dust with powdered sugar.
Serve immediately while they are still warm and the butter soaks in.