01 - Preheat oven to 375°F (190°C). Lightly grease a standard 6-cup muffin tin.
02 - In a small bowl, combine the softened butter, minced garlic, and chopped parsley until evenly blended.
03 - Flatten each bread slice with a rolling pin. Trim the crusts if required so each slice fits neatly into a muffin cup cavity.
04 - Spread a thin layer of garlic butter over one side of each bread slice. Place each slice, buttered side facing up, into a muffin tin well to form a cup.
05 - Distribute grated Parmesan cheese evenly across the bottom of each bread cup.
06 - Crack one large egg into each bread cup. Season lightly with salt and black pepper. If using, sprinkle shredded mozzarella or cheddar cheese over the eggs.
07 - Bake on the middle rack for 16 to 18 minutes, or until egg whites are set and yolks reach preferred doneness.
08 - Allow the egg cups to cool in the tin for 2 minutes, then gently lift out with a small spatula. Serve warm, garnished with extra parsley if desired.