Confectioners’ sugar for dusting
4 envelopes, 1/4 oz. each, unflavored gelatin
1 cup cold water, divided into 3/4 and 1/4 cups
2 cups granulated sugar
1/4 cup light corn syrup
1 tablespoon vanilla extract or other flavorings — see note
4 large egg whites, room temperature
Line a 9x13 pan or quarter sheet pan with parchment paper, waxed paper or plastic wrap. Let it hang over the sides to allow for easy removal of marshmallows.
Sprinkle generously with confectioners’ sugar.
Sprinkle gelatin on 3/4 cup water to rehydrate. Give it a stir. It will get fairly thick.
Combine sugar, corn syrup, vanilla, remaining 1/4 cup cold water and 4 egg whites to mixing bowl.
Whisk over simmering water (or use a double boiler) until mixture is hot and sugar has dissolved.
Stir in softened gelatin and whisk until gelatin dissolves.
Remove from heat and whip at medium speed until mixture rises in volume and begins to thicken. Turn speed to high and whip until mixture holds a soft, but stable, peak.
Spread in prepared pan and sift confectioners’ sugar all over the top.
Chill in refrigerator until fully set, about 2 hours.
Cut into 1” squares.
Store between layers of wax or parchment paper in airtight container. Keep in dry, cool place for up to 1 month.