In a small bowl, dissolve the active dry yeast and granulated sugar in the warm milk, letting it sit for about 5 to 10 minutes until it becomes frothy.

In a large mixing bowl, whisk together the all-purpose flour, buckwheat flour, and salt until well combined.

Add the egg, lukewarm milk, melted butter, and the activated yeast mixture to the dry ingredients, whisking continuously until a smooth batter forms without any lumps.

Cover the bowl with a clean kitchen towel and let the batter rest in a warm place for 45 minutes, or until it has doubled in size and is visibly bubbly.

Transfer the rested batter into a plastic squeeze bottle or a piping bag with a small opening to make pouring into the pan neat and easy.

Heat a poffertjes pan (or a non-stick takoyaki pan) over medium heat and brush each indentation generously with softened butter.

Squeeze the batter into each indentation, filling them about three-quarters of the way full so they have room to rise and puff up.

Cook for 2 to 3 minutes until the bottoms are golden brown and small bubbles begin to pop on the top surface of each pancake.

Use a wooden skewer or fork to quickly flip each mini pancake over, cooking the other side for an additional 1 to 2 minutes until fully puffed and golden.

Transfer the hot poffertjes to a warm serving plate, top with a pat of butter, and dust generously with powdered sugar before serving immediately.