Bring the milk to 90 to 100°F, mix in the yeast, and leave it for 5 minutes while measuring the dry ingredients. Keep the milk lukewarm, not steaming hot, to give the yeast suitable conditions.

Combine the flour, sugar, and salt with a whisk in a large bowl. An evenly blended dry mixture distributes sweetness and seasoning throughout the batter.

Add the yeast-milk mixture and the egg to the bowl, then stir only until no dry flour remains. Leave small lumps alone rather than working the batter until perfectly smooth.

Drape a clean kitchen towel over the bowl and allow a 1-hour rise at room temperature. Look for roughly twice the starting volume and a bubbly surface; a 30-minute rise is a compromise when pressed for time, but the full hour develops more flavor.

Preheat a poffertjes or aebleskiver pan for 3 minutes over medium to medium-high heat, then coat every cavity generously with butter. The butter should melt readily and cover the surfaces where the batter will touch.

Portion batter with a small cookie scoop until each cavity is about halfway full, adding warm milk to the bowl one tablespoon at a time only if needed. The mixture should fall from the scoop without needing to be scraped out as a firm lump.

Give the first side 1 to 2 minutes, then catch the edge with a wooden skewer and roll each pancake over within its cavity. Bubbles across the top and a golden underside signal the turn while the upper center remains soft.

Allow another 1 to 2 minutes for the second side to cook. Both faces should develop a golden-brown color, with a cooked, tender center rather than wet batter.

Remove the pancakes using the skewer or a small fork, then butter the empty cavities before adding the next batch. A fresh coating helps the next round release without tearing its delicate edges.