Preheat oven to 350°F. Using a serrated knife and cutting in same direction as you would to make a sandwich from a croissant, cut each croissant into about 5 (¼-inch-thick) slices. Place slices in a single layer on a parchment paper–lined baking sheet.
Stir together honey, sugar, 1 tablespoon water, and salt in a small saucepan. Cook over medium, swirling pan occasionally, until mixture is dark amber, smells toasty, and a candy thermometer inserted in mixture registers 320°F, 6 to 8 minutes. Remove from heat, and stir in orange zest.
Brush tops of croissant slices lightly with syrup. Reserve remaining syrup for another use (see Note). Top croissant slices with a piece of parchment paper and a second baking sheet; weight with a large cast-iron skillet. Place baking sheets with skillet on top in preheated oven, and bake until croissant slices are deep golden brown and have a strong caramel aroma, 18 to 22 minutes.
Uncover croissant slices, and, if desired, sprinkle with sesame seeds. Return croissant slices, uncovered, to oven. Turn off oven, and open door about 2 inches. Let tuiles cool in oven 15 minutes. Remove from oven, and let cool completely on baking sheet, about 30 minutes.
