Butter a 9x13-inch pan generously and dust bottom and sides with powdered sugar.

Put ⅓ cup water in a small bowl and sprinkle gelatin over top; set aside to soften.

In a heavy saucepan, heat remaining ⅓ cup water and sugar over low heat, stirring until sugar is dissolved. Increase heat to medium and let bubble, without stirring, until the mixture reaches 250F. Be patient, it can take 10+ minutes. Remove from heat and whisk in the gelatin mixture.

Put the egg white in the bowl of a stand mixer fitted with a whisk and beat to stiff peaks. Lower the mixer to low and slowly stream in the hot syrup. Beat until the mixture is thick and white and has tripled in size. Fold in vanilla and half of the sprinkles.

Pour mixture into the prepared pan and use a spoon to smooth the top. Sprinkle with remaining marshmallows.

Chill overnight.

To remove, run a knife around the sides of the pan and invert onto a powdered sugar covered cutting board. The easiest way to cut the marshmallows is with a pizza cutter. Toss them in powdered sugar after you cut them to make sure they don't stick to each other.

Keep in an airtight container for up to a week.