Lay four slices of bread on a clean work surface. Spread about 1 tablespoon of marinara over each slice, leaving a small border around the edges.

Divide the shredded mozzarella evenly between the four slices and arrange six pepperoni slices over each portion of cheese. Top with the remaining bread slices and press gently.

In a shallow bowl, whisk together eggs, milk, Parmesan, oregano, garlic powder, salt, and black pepper until well combined.

Dip each stuffed sandwich briefly into the egg mixture, coating both sides without allowing the bread to become overly saturated.

Heat a large nonstick skillet or griddle over medium-low to medium heat. Melt 1 tablespoon of butter in the pan.

Cook two sandwiches for 3–4 minutes per side until deep golden brown and the mozzarella has melted. Reduce the heat if the bread browns too quickly.

Transfer the cooked sandwiches to a plate. Add the remaining butter and repeat with the remaining two sandwiches.

Rest for 1 minute, then slice diagonally. Garnish with basil, Parmesan, or oregano if desired and serve immediately with warm marinara sauce for dipping.