Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper.
In a small saucepan, combine the milk, water and oil and bring to a boil.
In the bowl of a stand mixer fitted with the whisk attachment, combine the hot milk mixture with the tapioca flour and beat on medium speed. Add the mozzarella, Parmesan, eggs and salt and mix until just combined. The dough will be very soft; if necessary, add a little more tapioca flour.
Shape and roll the dough into about 24 balls with your hands; moisten your hands if the dough is very sticky. Transfer the balls to the prepared pan. Sprinkle the balls with a little more mozzarella and bake until golden and puffed, 15 to 20 minutes. Serve with marinara and/or chimichurri. Enjoy!