In a medium mixing bowl, combine the warmed milk, instant yeast, and sugar. Whisk thoroughly and allow the mixture to rest for approximately 5 minutes, or until the yeast becomes frothy.

In a separate large bowl, mix the all-purpose flour with the salt. Once the yeast mixture is ready, pour it into the bowl containing the dry ingredients, followed by the egg.

Using an electric hand mixer, blend the batter until smooth and lump-free. Alternatively, a whisk may be used, though the mixer ensures faster results.

Cover the bowl with a clean towel or plastic wrap, and let the batter rest for 45 minutes to 1 hour. During this time, the batter should rise significantly, becoming bubbly and nearly doubling in volume.

Preheat a poffertjes pan or a large nonstick skillet over medium heat. Lightly grease each indentation of the poffertjes pan or the surface of the skillet with softened butter.

Spoon a heaping tablespoon of batter into each cup of the pan or onto the skillet, ensuring adequate spacing if using the latter.

Cook the poffertjes for around 2 minutes, or until bubbles appear on the surface of the batter. Gently flip them using a skewer, fork, or spatula, and cook for an additional 1-2 minutes until both sides are golden brown.

Remove the poffertjes from the pan and repeat the process with the remaining batter.