Combine powdered sugar, cornstarch, and ¼ cup sprinkles (nonpareils).
Line an 8×8 pan with parchment paper, and coat the bottom and sides with approximately half of the cornstarch mixture. Set aside.
Put gelatin and first ½ cup of water into the mixing bowl of a stand mixer with the whisk attachment and set aside.
Heat sugar, corn syrup, and the other ½ cup water in a small to medium pot, covered over medium-high heat for approximately 5 minutes.
Uncover and continue heating until mixture reaches 240 F on a candy thermometer. This may take some time. Remove from heat.
Begin beating the gelatin mixture with the whisk attachment of the mixer and very slowly pour in the syrup mixture.
Increase the mixer to high speed (do this slowly to avoid splatter!) and beat for an additional 10 minutes.
Beat in vanilla till just mixed.
Stir in ¼ cup sprinkles (jimmies).
Pour mixture into the prepared pan and spread evenly with a greased spatula.
Sprinkle the remainder of the powdered sugar mixture over top and let sit for at least 4 hours. Overnight is ideal.
Cut into cubes with a greased knife. Roll the sides of each marshmallow cube in the excess corn starch mixture and enjoy!