Line two pans, 8×8″ or bigger, with parchment paper, leaving an overhang on two sides (like handles). Brush the bottom and sides of the pans with vegetable oil; dust with powdered sugar. Set aside.
In a small pot, simmer the cider until it’s reduced down to 1/2 cup. Pour into a bowl to cool. When it’s no more than warm, whisk in the gelatin.
Meanwhile, combine 2 cups water and granulated sugar in a medium pot. Stir over low heat until the sugar dissolves, then heat to a rolling boil until the temperature reaches 250F.
Meanwhile, beat the egg whites until stiff.
Whisk the cider-gelatin mix into the sugar mixture (if the gelatin has set, microwave it gently). Turn the mixer on low speed and slowly pour in the sugar-gelatin mix. Increase speed to medium high and beat until the mixture is doubled in volume and falls in thick ribbons when the whisk is raised, about 5 minutes.
Pour the mix into the pans. Allow to set at room temperature, at least 2 hours.
To remove marshmallows, use the parchment paper handles to lift the contents out. Cover a baking sheet with more powdered sugar. Cut marshmallows to desired size and place on baking sheet. Toss until they are well coated.
Store at room temperature in an airtight container for up to 2 weeks.