Butter the bottom and sides of an 8 by 8-inch straight-sided baking pan. Dust liberally with sifted powdered sugar.
Pour 1/2 cup water into the bowl of a stand mixer fitted with the whisk attachment. Sprinkle the gelatin over the water and allow to soften, about 10 minutes.
In a 3-quart saucepan, combine the sugar, evaporated milk and 1/4 cup water. Stir over low heat until the sugar has dissolved. Bring the mixture to a simmer and cook, without stirring, until a candy thermometer registers 240 degrees F, about 10 to 12 minutes. Pour the syrup over the gelatin mixture. Beat the mixture at low speed until incorporated. Increase the speed to high and beat until thick, fluffy and tripled in volume, about 10 to 12 minutes. Beat in the orange zest and vanilla extract until just combined. Using a spatula, scrape the mixture into the prepared pan and smooth the surface. Sprinkle with orange decorating sugar. Allow the mixture to set, uncovered, in a cool place (not refrigerated) for at least 8 hours or overnight.
To make orange ganache:
Place finely chopped chocolate in small heatproof bowl.
Set over warm water on low heat (water should not touch bottom of bowl).
Stir very frequently until almost melted. Remove from heat and hot water; stir until smooth, pressing out any lumps with back of spoon or a rubber spatula.
In small saucepan over low heat, heat cream just to a simmer, stirring frequently.
Remove from heat; all at once, add about two-thirds of hot cream to melted white chocolate.
Gently stir to incorporate, then beat or whisk until smooth, scraping bowl as necessary with rubber spatula.
Gradually stir in remaining cream.
Cut marshmallows into squares matching same dimensions as the graham crackers.
place a cut marshmallow on top of one graham cracker.
spread a layer of the orange ganache on the 2nd graham cracker.
lay that on top of the marshmallow, press down gently.
dip one corner half of the s'more into orange candy melt.