In a large mixing bowl, whisk together the flour, sugar, and salt until combined. Set aside.
Heat the milk in the microwave until it reaches 110°F, about 60-90 seconds, or heat it in a small saucepan over low heat. Remove from the heat. Gradually whisk the active dry yeast into the warm milk until dissolved. Cover and rest for 10 minutes, until a thick, frothy layer develops on top.
Add the yeast mixture to the dry ingredients, followed by the eggs and vanilla extract. Mix just until combined. Do not overmix.
Cover the bowl with a lid or clean tea towel and place it in a warm area for 30 minutes. I recommend placing the bowl in a kitchen sink filled halfway with warm water. The batter should become thick, bubbly, and frothy.
Heat a poffertjes, Aebleskiver, or Takoyaki pan over medium-low heat. Lightly grease the wells with melted butter. Spoon about 1 tablespoon (15 ml) of batter into each well, filling it halfway. If using a large non-stick pan, lightly grease it with melted butter and drop 1 tablespoon (15 ml) of batter per pancake into the pan.
Cook until bubbles form on the surface and the bottoms are golden brown, about 2-4 minutes. Flip each pancake with a wooden skewer or fork and cook the other side until golden, about 2-3 minutes. Transfer to a serving plate and serve warm with powdered sugar or your favorite toppings.