To make Chocolate Crunchy Cornflake Layer: Assemble 9x23/4-inch nonstick springform pan with bottom insert turned over so lip of insert is facing down. Place 4 cups cornflakes and chopped peanuts in large bowl, and set aside.
Melt semisweet chocolate and butter together over low heat. Pour mixture over cornflakes and peanuts. Use rubber spatula to stir ingredients until coated; the cornflakes will crush in the process. Transfer mixture to springform pan. Firmly and evenly spread to inside edges using rubber spatula. Refrigerate.
To make Chocolate, Peanut and Grape Layer: Place semisweet chocolate in medium bowl. Heat heavy cream, butter and sugar in saucepan over medium-high heat. When hot, stir to dissolve sugar. Bring to a boil. Pour boiling cream over chopped chocolate. Set aside for 5 minutes, then stir with whisk until smooth. Add grapes and 1 cup peanuts, and stir to combine. Pour mixture over cornflake layer, spreading evenly. Sprinkle remaining peanuts over surface. Refrigerate for 12 hours before cutting.
To serve, cut around inside edge of pan with thin-bladed paring knife, then release springform sides. Heat blade of serrated knife under hot running water, and wipe blade dry before cutting each slice. Keep slices at room temperature for 20 to 30 minutes before serving.