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1
Prepare an 8- by 8-inch pan by evenly greasing the bottom and sides with butter. Set aside.
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2
To assemble bottom layer: Melt 7 ounces of chocolate in the microwave. First, break the chocolate into 1-inch pieces and put them in a glass bowl. Microwave on 50% power for about 1 minute. At this point, the chocolate may look like it has not started to melt. Stir well, then microwave the chocolate in short intervals (about 15 to 30 seconds), stirring well between each interval, until it's melted. Let cool to room temperature.
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3
Meanwhile, in the bowl of a stand mixer, combine peanut butter, Nutella, and oil. Beat on medium-high speed until combined and lighter in color, 2 to 3 minutes. Reduce speed to low and add the cooled melted chocolate until thoroughly combined. Remove bowl from mixer and fold in the puffed rice cereal. Spread evenly into prepared pan with spatula. Cover with plastic. Freeze until firm, about 1 to 2 hours.
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4
To assemble top layer: Melt the remaining 6 ounces of chocolate in microwave, then remove and stir in a pinch of sea salt and the small pieces of butter until evenly incorporated. If butter doesn't completely melt and integrate, return to microwave for about 15 seconds. Set aside to cool.
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5
In a large mixing bowl, whip cream to soft peaks. Refrigerate until ready to use.
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6
Combine eggs whites and sugar in bowl of a stand mixer. Place the bowl over a pan of simmering water, making sure the bottom of the bowl doesn't touch the water. Whisk gently until sugar dissolves and the egg whites are hot. (If you're concerned about undercooked eggs, whisk until a candy thermometer inserted into mixture registers a temperature of 160u00b0 F, then immediately remove from heat.)
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7
Transfer mixer bowl to mixer fitted with a whisk attachment and beat on medium high speed until mixture cools, doubles in volume, and forms stiff peaks, about 10 to 12 minutes.
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8
Fold together chocolate mixture and meringue quickly yet gently. If the mixture is any warmer than room temperature, let it cool for a few minutes. Then, fold in the whipped cream.
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9
Remove pan from freezer and spread mousse layer evenly across the bottom layer using a spatula. Cover with plastic wrap, making sure it fits tautly across the top. (I like to cover with a piece of foil as well.) Freeze until firm, another 3 to 4 hours.
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10
When ready to serve, remove the pan from the freezer and let sit for about 5 to 10 minutes to soften slightly. Cut into 16 1-inch bars with sharp knife. If needed, dip the knife into warm water, then dry, between each slice. Alternatively, if serving a crowd, run your knife around the edge of the semifreddo, and lift (using a large flat spatula if needed) the entire dessert from the pan on to a cutting board, then slice in same manner. Dust the tops of the bars with cocoa powder, and serve with chopped hazelnuts and red raspberries on the side if desired. (The taste won't suffer if you need or want to skip the garnishes.)
