Bloom gelatin: sprinkle 3 packets unflavored gelatin (about 2 1/2 Tbsp / ~21 g) over 1/2 cup (120 ml) cold water in a bowl; let stand 5–10 minutes
Prepare syrup: combine 1 cup (200 g) sugar, 1/2 cup (120 ml) light corn syrup, and 1/4 cup (60 ml) water in a saucepan
Heat syrup: bring to a boil over medium-high heat, stirring until sugar dissolves
Cook to soft-ball stage: continue boiling until it reaches 240°F (115°C) on a candy thermometer
Add gelatin mixture: remove syrup from heat; pour in bloomed gelatin while whisking on low speed
Whip: increase to high speed and beat until very thick and tripled in volume, 8–12 minutes
Add flavor and salt: add 1–2 tsp vanilla extract and a pinch of salt near the end
Prepare pan: lightly oil or spray a 9×13 in (23×33 cm) pan; dust with powdered sugar or cornstarch
Set: scrape marshmallow mixture into the pan; smooth top
Rest: let set uncovered at room temperature 4–6 hours or overnight
Cut: dust knife and cut into squares or use a greased cutter
Coat: toss pieces in powdered sugar/cornstarch to prevent sticking
Store: keep in an airtight container at room temperature up to 1 week