In the microwave or a pan, heat milk until slightly warm (just above lukewarm). Then, in a small bowl, mix together the milk, sugar, and active dry yeast. Set the mixture aside for 10 minutes, until mixture looks bubbly and foamy.
Meanwhile, in a large mixing bowl, combine flour and salt.
Add the egg and the liquid mixture to the dry ingredients. Mix with a hand whisk or an electric mixer until you have a smooth, silky batter.
Cover with a clean, damp cloth and set aside for one hour, or until the batter has about doubled in size.
Transfer the batter into a piping bag or a squeeze bottle. Grease your poffertjes pan with butter and heat it over medium heat. Once the pan is hot, fill up each poffertjes mold with batter, then cook each side for one minute, using a fork or butter knife to flip.
Serve your poffertjes hot, with butter, powdered sugar, and any other toppings you wish!