1. Peel the yuca and cook in boiling water until tender. Drain, mash and season to taste with salt and pepper. Beat in the butter; add the milk to the mixture and stir until it has the consistency of mashed potatoes. Beat in the egg yolks one by one.
2. Beat the egg whites with a pinch of salt until they hold stiff peaks, fold into the yuca mixture and pour into a 2-quarts souffle dish. Bake in a preheated moderate 3504 oven for 35 minutes or until puffed and browned.