Firstly, make sure your milk is lukewarm. I like to use the microwave and heat in 30-second increments. You just want it warm so if you hold your hand against it, it’s not too hot or too cold. If it is too hot, let it cool slightly or it will kill the yeast. If you have a thermometer, it should be between 38-46°C.
In a large jug, mix together the lukewarm milk, yeast and sugar. Leave to sit for about 10 minutes - or until frothy and bubbly. Technically, you don’t need to do this with fast action yeast, but I always like to check to ensure it is active and working.
Next, add the flour and salt to a large mixing bowl. Pour in the milk, followed by the egg, then whisk well until you have a smooth batter.
Cover with cling film or a clean tea towel and set aside to rest in a warm place for about 45 minutes to 1 hour - or until really bubbly and roughly doubled in size. If your house is colder, it might take a little longer. Make sure it doesn’t over-prove or it will develop a sour flavour.
When the poffertjes batter is ready, place a poffertjes pan or large non-stick frying pan over a medium heat. Once hot, grease generously the cavities of the pan with butter (or the base of the non-stick frying pan) - either by brushing with a pastry brush or piece of buttered paper.
Pour the poffertjes batter into a squeezy bottle (or use a small cookie scoop, ladle, piping bag or jug) and fill each hole of the poffertjes pan with batter until nearly at the top. If you don’t have a poffertjes pan, you can just squeeze out small rounds (you want to use around 1 tbsp of batter for each one - depending on the size you want your poffertjes). Cook for a few minutes or so - or until bubbly on top and golden underneath (use a toothpick to gently lift up and check). Quickly and carefully, flip the poffertjes over and cook for another minute or so on the other side. The exact time it takes to cook each side will depend on a) the amount of batter and b) the heat of your pan. Watch closely and adjust the heat/cooking time accordingly - take care not to overcook. Remove from the pan and place onto a heatproof dish or plate. Cover with foil and place in an 100c oven to keep warm. Repeat with the remaining batter until you have around 50 poffertjes.
Serve the poffertjes warm topped with a knob of butter, a dusting of sieved icing sugar and fresh raspberries.