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1
Combine egg whites, sugar, and salt in the heatproof bowl of a standing mixer set over a pan of simmering water. Whisk constantly by hand until mixture is warm to the touch and sugar has dissolved (the mixture should feel completely smooth when rubbed between your fingertips).
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2
Attach the bowl to the mixer fitted with the whisk attachment. Starting on low and gradually increasing to medium-high speed, whisk until stiff (but not dry) peaks form. Continue mixing until the mixture is fluffy and glossy, and completely cool (test by touching the bottom of the bowl), about 10 minutes.
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3
With mixer on medium-low speed, add the butter, a few tablespoons at a time, mixing well after each addition. Once all butter has been added, whisk in vanilla. Switch to the paddle attachment, and continue beating on low speed until all air bubbles are eliminated, about 2 minutes. Scrape down sides of bowl with a flexible spatula, and continue beating until the frosting is completely smooth. Keep buttercream at room temperature if using the same day, or transfer to an airtight container and refrigerate up to 3 days or freeze up to 1 month. Before using, bring to room temperature and beat with paddle attachment on low speed until smooth again, about 5 minutes.
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4
Combine chocolate and oil in a medium heat-proof bowl set over a medium saucepan of barely simmering water; stir until melted and smooth. Transfer to a small bowl, and let cool about 15 minutes.
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5
Holding each cupcake by its bottom, dip cupcake in the chocolate to coat frosting, allowing excess to drip off. Transfer to a baking sheet fitted with a wire rack. Spoon more coating around edge of cupcake and any exposed frosting; none of the frosting should show. ( I actually liked having some of the frosting show. ) Let cupcakes stand at room temperature 15 minutes.
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6
Carefully remove paper liners from cupcakes, and discard. Place cupcakes on a serving platter, and refrigerate for 30 minutes to let coating set. Cover, and refrigerate for 2 hours more. Serve cold. Cupcakes can be refrigerated for up to 3 days.
