In a mixing bowl, whisk together eggs, milk, vanilla extract, cinnamon, and any optional add-ins.
Slice croissants in half lengthwise.
Dip each croissant half into the egg mixture, making sure to coat both sides without oversoaking.
Heat a nonstick skillet or griddle over medium heat and melt the butter.
Place the soaked croissants on the skillet and cook for 2-3 minutes per side, or until golden brown and crispy.
Transfer to a serving plate and repeat with the remaining croissants.
Top with powdered sugar, fresh berries, whipped cream, or your favorite toppings.
Drizzle with maple syrup and serve immediately.