Preheat your oven to 350°F (180°C) and line a 12-hole cupcake pan.
In a large bowl, cream together the butter and both sugars until pale and fluffy.
Beat in eggs one at a time, then stir in the vanilla.
Sift in flour, cocoa, and baking powder. Add buttermilk and hot water. Mix until smooth.
Divide batter evenly among liners. Bake for 18–20 minutes. Cool completely.
In a heatproof bowl over simmering water, whisk egg whites, sugar, and cream of tartar for 4 minutes.
Transfer to a stand mixer. Beat until stiff peaks form. Add vanilla.
Pipe tall swirls on cooled cupcakes. Freeze for 15–20 minutes until firm.
Melt chocolate and oil until smooth. Let cool slightly.
Dip each cupcake upside down into the chocolate shell. Set upright and chill until set.