- Peel the cassava (yucca) root under cold running water, as it discolors quickly.
- Slice it and drop it into a saucepan of salted water. Bring to a boil, lower the heat, cover, and simmer until tender, about 30 minutes.
- Drain the cassava, mash, season to taste with salt and pepper, and beat in the butter.
- Heat the milk and stir it gradually into the mashed vegetables until it has the consistency of mashed potatoes.
- Use a little more hot milk if necessary.
- Beat in the egg yolks one by one.
- Beat the egg whites with a pinch of salt until they stand in firm peaks.
- Fold them into the vegetable mixture lightly but thoroughly and pour into a 2-quart souffle dish.
- Bake in a preheated moderate (350 °F) oven for 35 minutes or until well puffed and lightly browned.
- Leave whole or cut into wedges and serve immediately.