Melt 1 tablespoon butter in a large skillet over medium-high heat. Add pear slices and 1 tablespoon sugar; cook, stirring occasionally, until pears are tender and caramelized, about 10 minutes.

Put eggs, milk, orange juice, eau de vie, if desired, lemon juice, remaining 2 tablespoons sugar, salt, and zests in a wide, deep dish. Beat well with a fork.

Dip brioche slices into egg mixture; transfer to a rimmed baking sheet. Divide pears among half the slices. Top with remaining slices.

Melt remaining 3 tablespoons butter in a large nonstick skillet over medium heat. Transfer 2 or 3 pear sandwiches to skillet with a spatula. Cook until deep golden brown on one side, about 2 minutes. Turn; cook until deep golden brown and slightly puffy all over, about 2 minutes more. Repeat with remaining sandwiches. Serve immediately with maple syrup or confectioners' sugar, if desired.