Nonstick cooking spray
2 cups granulated sugar
1 teaspoon fine salt
6 tablespoons unsalted butter
1 cup heavy cream
3 1/2 cups mini marhsmallows, 8 ounces
1 teaspoon pure vanilla extract
3 cups coarsely chopped dark or white chocolate, 20 ounces
2 tablespoons sanding sugar
Cut three 3-by-18-inch strips waxed paper; line 9-by-12-inch rimmed baking sheet, 1 strip along length and 2 along width. Allow strips to hang over sides. Coat with cooking spray; set aside. In medium saucepan, combine granulated sugar, salt, butter, cream, and marshmallows. Cook, stirring occasionally, over medium-low heat until most of the butter and the marshmallows are melted, 4 to 5 minutes. Bring to boil; boil, stirring, 5 minutes. Remove from heat. Add vanilla and chocolate; stir until completely smooth. Pour immediately into prepared baking sheet; let cool, 3 to 4 hours.
When fudge is firm enough to work with, use paper to lift fudge onto a cutting board. Using 1-, 1 1/4 -, and 1 1/2-inch round cutters, cut out 14 rounds of each size. Pour sanding sugar into a small, shallow bowl; dip top of each round in sugar. Stack rounds to form 3-layer cakes; let stand until fudge is completely solid, about 1 hour.