Boiling yuca: Cut the cassava into small pieces and boil until it is cooked through, after adding salt to the water. Remove from the water and discard the water.
Combining: Mix yuca with milk, butter, and egg, then mash until it becomes a soft puree (if you have a potato ricer [Amazon affiliate link] use it to obtain a smoother puree). Set the cassava puree aside.
Assembling: Grease a mid-sized baking pan 8 x 8 inches [20 x 20 cm] with the butter. Spread half of the mashed yuca, smoothing it out to obtain a level layer. Top with half the mozzarella, follow with the chicken filling, spreading into a level layer. op with the remaining mashed yuca. Spread and smooth it. Sprinkle with the rest of the mozzarella.
Cooking: Bake in preheated oven at 300 ºF [160 ºC] for 35 minutes, or until the cheese in the edge turns golden brown.
Serving: Remove from the oven and serve hot, best accompanied with a salad.