Bake the Devil's Food Cake, following instructions for cupcakes. Let cool.
Make the frosting: In the bowl of an electric mixer, combine sugar, egg whites, water, and cream of tartar. Whisk until foamy. Set bowl over a pan of barely simmering water and whisk until sugar is dissolved and mixture is warm to the touch, about 3 to 5 minutes. Transfer bowl to stand mixer fitted with a whisk attachment. Beat on high speed until frosting forms stiff peaks, about 12 to 16 minutes. Add vanilla and beat until combined.⏱ 16 min
Transfer frosting to two large pastry bags fitted with 1/2-inch round tips. Leaving a 1/8-inch border on each cupcake, pipe a spiral of frosting into a 2-inch-high cone shape, using about 1/3 to 1/2 cup frosting per cupcake. Transfer cupcakes to a baking sheet, and refrigerate while preparing the chocolate coating.
Make the chocolate coating: Combine chocolate and oil in a medium heat-proof bowl set over a saucepan of barely simmering water; stir until melted and smooth. Transfer to a small (deep) bowl, and let cool about 15 minutes.⏱ 15 min
Holding each cupcake by its bottom, dip cupcake in the chocolate to coat frosting, allowing excess to drip off. Transfer to a baking sheet filled with a wire rack. Spoon more coating around edge of cupcake and any exposed frosting; none of the frosting should show. Let cupcakes stand at room temperature 15 minutes.⏱ 15 min
Place cupcakes on a serving platter, and refrigerate for 30 minutes to let coating set. Cover, and refrigerate 2 hours more. Serve cold. Cupcakes can be refrigerated up to 3 days.⏱ 150 min