For smooth slices, dust your knife and the individual marshmallows in powdered sugar between each cut.
Line bottom of 7- x 11-inch baking dish with parchment paper; lightly grease with cooking spray. Dust sides and bottom of dish with powdered sugar. Sprinkle gelatin over 1/2 cup cold water in bowl of a heavy-duty electric stand mixer.
Stir together granulated sugar, next 2 ingredients and remaining 1/2 cup cold water in a medium saucepan over medium-high heat. Cook, stirring often, until a candy thermometer registers 240° (about 12 to 15 minutes; lower heat as necessary to prevent mixture from boiling over).
Gradually add hot sugar mixture to gelatin mixture, beating at medium-low speed and using whisk attachment. Gradually increase speed to high, and beat 7 minutes or until bottom of bowl is slightly warm and mixture is very thick. Stir in vanilla.
Working quickly, spread mixture in prepared dish; smooth with a lightly greased (with cooking spray) offset spatula.
Heavily dust mixture with powdered sugar. Let mixture stand, uncovered, in a cool, dry place 8 to 24 hours. (Mixture should be dry enough to release from baking dish.) Cut mixture into 1 1/2-inch squares.
Peppermint Marshmallows: Substitute 1 tsp. peppermint extract for vanilla extract. Prepare recipe as directed through Step 3, adding 4 drops red food coloring gel and swirling into marshmallow mixture, using a lightly greased rubber spatula. Spread in prepared dish, and dot with 6 drops red food coloring gel. Swirl, using a wooden skewer. Proceed with recipe as directed in Step
Bourbon-Caramel Marshmallows: Substitute 2 Tbsp. bourbon for vanilla extract. Prepare recipe as directed through Step 3, adding 2 Tbsp. warm jarred caramel topping and swirling into marshmallow mixture, using a lightly greased rubber spatula. Spread in prepared dish; drizzle with 2 Tbsp. warm jarred caramel topping. Swirl using a wooden skewer. Proceed with recipe as directed in Step