Thumbprint cookies may develop cracks when you press to create an indentation, especially if the dough has started to dry out or stiffen. To avoid this, make the indentation right after shaping each dough ball. If you notice a slight crack, reroll the dough ball it often softens and warms in your hands. If it continues to crack then add some additional melted butter to the dough 1 tsp at a time until the dough is more workable.