Preheat the oven to 220 degrees C. Line two 12 hole muffin trays with baking paper (use one oversized piece for each tray and push the baking paper into the holes).
In a large bowl whisk the milk, ricotta, parmesan, eggs, egg whites, salt and pepper. Stir through the baby spinach. Use a ladle or transfer mixture to a jug to fill the muffin holes lined by the poked-down baking paper.
Bake in preheated oven for 10-15 minutes or until just set. Cool until warm on benchtop then refrigerate in trays overnight. Gently peel frittatas away from the baking paper before serving. This can take up to 10 minutes, so make sure you leave time before your guests arrive.