How to Make It
Place 1/2 cup water in a small microwave-safe bowl; sprinkle gelatin over top. Set aside.
Bring remaining 1/2 cup water, granulated sugar, corn syrup, and salt to a boil in a medium saucepan over medium-high. Cook 16 minutes or until syrup mixture reaches 250°F, slowly swirling pan occasionally. Carefully pour syrup mixture into the bowl of a stand mixer fitted with a whisk attachment. Let stand 5 to 10 minutes or until mixture cools to 210°F.
Microwave reserved gelatin mixture at HIGH 30 seconds or until gelatin melts, stirring until smooth. Beat syrup mixture in stand mixer at low speed, gradually adding gelatin mixture in a thin, steady stream. Beat until combined. Add vanilla. Increase speed to high; beat 5 minutes or until light and fluffy. Add crushed wafer cookies; beat 10 to 20 seconds or just until combined.
Coat a spatula with cooking spray. Use spatula to scrape mixture into a lightly greased 11 x 7-inch straight-sided metal baking dish. Lightly coat hands with cooking spray and press mixture into an even layer.
Sift powdered sugar and cornstarch together onto a rimmed baking pan. Lightly coat a work surface with additional powdered sugar. Turn marshmallow onto work surface; cut into 78 (1-inch) cubes. Roll each cube in cornstarch mixture. Store marshmallows in an airtight container for up to 1 week.