In the bowl of a food processor, add the flour, starch, baking powder, and salt. Pulse briefly just to combine.
Add the egg, butter, queso fresco, and mozzarella. Process until smooth, approximately 2 minutes. The dough should come together in a ball. If it doesn’t come together, add the milk one tablespoon at a time and pulse again.
Form the dough into golf ball-sized spheres. Place them on a parchment-lined sheet pan about 2-inches apart and refrigerate for 30 to 60 minutes.
Preheat the oven to 500°F.
Place the baking sheet with the balls of dough in the oven and bake for approximately 7 to 10 minutes until the rolls are lightly golden brown.
Allow the rolls to rest for a couple of minutes until cool enough to handle, then serve while still warm.