In a saucepan, bring the butter, milk, and water to a boil.
Once boiling, add a pinch of salt and the flour all at once. Stir vigorously to prevent lumps from forming. Cook the mixture for about 1–2 minutes until it forms a smooth, compact dough and a white film appears at the bottom of the pan.
Allow the dough to cool slightly before adding the eggs one at a time. Use an electric mixer to work the dough until it is creamy and smooth.
Let the dough rest in the refrigerator for about 30 minutes.
Using a cooking thermometer, heat oil in a deep fryer or heavy pot to a maximum of 165 °C (329 °F). This temperature ensures the Sfinci cook thoroughly without burning.
Scoop about 30 g of dough with a spoon, using another spoon to help drop it into the hot oil. Carefully spoon some hot oil over the tops of the dough to help them expand.
Fry a few pieces at a time for about 2–3 minutes, turning them frequently so they triple in size and cook evenly. Occasionally tap them gently with a slotted spoon to encourage expansion.
Once golden brown, remove them from the oil and place them on paper towels to absorb excess oil. Allow them to cool before filling.