In a mixing bowl whisk together the eggs, cream, orange juice, zest, cinnamon, nutmeg, vanilla, sugar, and salt.

Lay the brioche slices flat on a clean work surface. Spread 1 side of each slice with 2 teaspoons of the mascarpone. Spread 2 teaspoons of the marmalade over the mascarpone on half of the slices. Place the slices together to form 4 sandwiches.

Working 1 at a time, dip the sandwich in the egg mixture, letting it sit about 1 1/2 minutes per side. Transfer to a plate and repeat with the remaining sandwiches.

Preheat a large nonstick skillet over low heat. Add 1 tablespoon of the butter. When the butter has melted, add 2 of the sandwiches and cook until golden brown on both sides, about 5 minutes per side. Repeat with the remaining butter and sandwiches.

Cut the French toast "sandwiches" in half and serve hot, dusted with powdered sugar and drizzled with maple syrup, as desired.