In the bowl of a stand mixer, dissolve the yeast in warm water.
Add the melted butter, sugar and let it sit for 5 minutes.
Add the egg, salt, and enough flour to form a soft dough.
Knead until the dough is smooth and elastic, about 3-5 minutes. Do not let it rise.

Divide the dough into 12 pieces and shape them into balls.

Place them on two baking sheets lined with parchment paper, leaving 3 inches (7.5 cm) of space between them.

Cover and let it rest for 30 minutes.
Brush with egg yolk mixed with milk.
Bake in a preheated oven at 425 F (220 C) for 8-12 minutes, or until golden brown.
Remove from the oven and let it cool on a wire rack.