Mini Potato Leek Frittata

Preheat oven to 375°F. Grease 12 muffin cups.
Cut potatoes into 1/2-inch chunks. In a medium saucepan, place potatoes, 1/8 teaspoon salt, and enough water to cover; heat to boiling over high heat. Reduce heat to medium and cook for about 10 minutes to partially cook potatoes (a fork should be able to go through them). Drain potatoes in a colander and set aside to cool.
Cut leeks lengthwise in half and clean thoroughly under cold running water. Pat dry, then slice crosswise into 1/4-inch slices. In a medium skillet, heat olive oil over medium-low heat; add leeks and cook until fragrant and wilted, about 10 minutes, stirring occasionally. Remove from heat; cool.
Into a large bowl, carefully crack eggs. Add egg whites, milk, pepper, and remaining 1/8 teaspoon salt, and whisk until mixed. Add cooled leeks, potatoes, cheese, and herbs and fold ingredients together until combined.
Ladle the frittata mixture evenly into prepared muffin cups (filling will come almost to the top). Set the pan in the middle of the oven. Bake for 14–16 minutes or until egg mixture puffs up and knife inserted in frittatas comes out clean. Cool for 1 minute then loosen edges of frittata with a sharp knife and scoop them out of the pan one by one. (They will deflate somewhat after cooling.)
Serve the frittatas warm or at room temperature.