Caramel Corn: Preheat oven to 225 degrees. In a large heatproof bowl, sprinkle peanuts over popcorn.

In a saucepan, bring brown sugar, butter, corn syrup, and salt to a boil over medium-high heat, stirring, until smooth and combined. Cook until darkened slightly and fragrant, about 3 minutes more. Remove from heat; immediately stir in baking soda and vanilla. Pour over popcorn mixture, stirring to evenly coat. Transfer to a parchment-lined rimmed baking sheet.

Bake 1 hour. Let cool completely. Break into bite-size pieces and serve, or store in an airtight container at room temperature up to 3 days. (Makes 10 cups.)

Semifreddo: Line the bottom and sides of a standard 9-by-5-inch loaf pan with plastic wrap, leaving a 2-inch overhang on long sides. In a large heatproof bowl, combine chocolate, 1/2 teaspoon vanilla, and 1/4 teaspoon salt. In another large heatproof bowl, combine peanut butter and remaining 1/2 teaspoon vanilla and 1/4 teaspoon salt. In a saucepan, bring cream to a boil. Remove from heat; immediately divide cream evenly between bowls. Let stand 5 minutes, then stir each mixture until smooth. Let cool until thickened slightly, about 15 minutes.

Combine egg whites and sugar in the heatproof bowl of an electric mixer set over (not in) a pot of simmering water. Beat constantly until sugar dissolves and mixture is warm to the touch (it should feel smooth when rubbed between fingers), 2 to 3 minutes. Remove from heat, then transfer to mixer fitted with the whisk attachment; beat on low speed until foamy. Increase speed to high; continue beating until stiff, glossy peaks form and bowl no longer feels warm to the touch, 7 to 9 minutes.

Divide meringue evenly between chocolate and peanut-butter mixtures; gently fold into each until thoroughly combined and no white streaks remain. Transfer half of chocolate mixture to prepared loaf pan; smooth top with an offset spatula. (Refrigerate peanut-butter mixture and remaining chocolate mixture until ready to use.) Freeze until thickened slightly, about 15 minutes. Pour peanut-butter mixture into pan, smoothing top; freeze 15 minutes. Pour remaining chocolate mixture over peanut-butter mixture and smooth top. Gently fold plastic-wrap overhangs over semifreddo to cover. Freeze until firm, at least 12 hours and up to 3 days.

Remove semifreddo from freezer 10 minutes before serving. Remove plastic from top, flip pan onto a chilled serving platter or cutting board, and lift pan from plastic. Remove plastic and slice semifreddo crosswise into 1-inch pieces. Serve, with caramel corn. Semifreddo can be stored, covered, in freezer up to 3 days.