In a mixing bowl, combine the mascarpone cheese, powdered sugar, and vanilla extract. Mix until smooth and well-combined. Set aside.
Take four slices of bread and spread a generous amount of the mascarpone mixture on each slice. Top with the remaining four slices to create sandwiches.
In a shallow bowl, whisk together the eggs, milk, and ground cinnamon until fully combined.
Heat a frying pan or skillet over medium heat and add 1 tablespoon of butter.
Once the butter is melted and sizzling, dip each sandwich into the egg mixture, coating both sides well.
Place the coated sandwiches in the pan and cook for about 3-4 minutes on each side, or until golden brown and cooked through. Add more butter if needed while cooking.
While the French toast is cooking, prepare the caramelized peaches. In another pan, melt the remaining tablespoon of butter over medium heat. Add the peach slices, brown sugar, and lemon juice. Cook for about 5-6 minutes until the peaches are softened and caramelized.
Once the French toast is done, serve it on plates topped with the caramelized peaches.