Make first caramel layer: Coat a rimmed baking sheet with cooking spray. Line with parchment, leaving a 2-inch overhang on 2 sides, then coat parchment with spray.
Place 2 cups cream, 2 cups sugar, 1 cup corn syrup, and 6 tablespoons butter in a large pot. Bring to a boil, stirring until sugar dissolves and butter melts, about 5 minutes. Cook over medium heat, stirring often, until mixture registers 245 degrees on a candy thermometer, about 20 minutes. Remove pot from heat, and stir in 3/4 teaspoon salt and 1/2 teaspoon vanilla. Pour into prepared pan without scraping bottom of pot. Let stand.
Make the marshmallow layer: Sprinkle gelatin over 1/3 cup water in a mixer bowl, and let soften, about 5 minutes.
Mix sugar and remaining cup water in a small saucepan over medium-high heat, stirring until sugar dissolves. Wash down sides of pan with a wet pastry brush to prevent sugar crystals from forming. Cook, undisturbed, until mixture registers 238 degrees on a candy thermometer, about 10 minutes. (You may have to tilt pan to get an accurate read.) Remove pan from heat, and whisk sugar syrup and a pinch of salt into gelatin. Pour into a mixer bowl, and whisk on medium-high until cool and thick, about 10 minutes. Spread marshmallow evenly over caramel to cover, and let stand for 30 minutes.
Prepare second caramel layer with remaining ingredients as in step 2. Pour over marshmallow layer, covering entire surface, and let stand, uncovered, until set, about 8 hours.
Using a 1 1/4-inch round cutter, cut out 70 rounds. Using a fork, dunk each round in chocolate (pictured, page 228). Scrape bottom of fork against edge of bowl to remove excess, and place dipped candies on a parchment-lined baking sheet. Sprinkle nonpareils on top, and let stand until set. Caramallows will keep, covered and refrigerated, for up to 5 days
These indulgent treats include two caramel layers, and each layer must be made separately at different times. Start the second one after the marshmallow layer has set on top of the caramel.