Add the flour, baking powder, sugar and salt to a large bowl. Make a well in the centre and crack in the eggs. Whisk in gradually, taking in some of the flour from the edge of the well. Slowly add the milk, whisking constantly until you have a smooth, slightly thick batter. Transfer to a squeezy bottle or measuring jug.
Preheat the poffertje pan over a low-medium heat. Grease each of the grooves with a small bit of butter, it’s easiest to do this with a little stick of butter on the end of a knife so you can move quickly without moving the pan.
Add enough batter to come ¾ of the way up each groove and cook until the tops are mostly dry and bubbly on the first side, around 2 minutes, and then flip using your skewer in a swift, decisive motion. Cook for a further minute on the second side, then use your skewer to pierce all the cooked poffertjes and move them on to your serving plate.
Slather in butter and sprinkle with icing sugar, eat immediately.