Preparation

Make the cupcakes:

• Preheat the oven to 350°F, with a rack in the middle. Line 16 cupcake cups with liners.
• Add the milk to a small saucepan. Split the vanilla bean lengthwise and scrape the seeds into the milk with the side of a paring knife. Add the vanilla pod and bring the milk to a simmer, then remove from the heat. Cover the saucepan and let steep for 10 minutes. Discard the vanilla pod and cool the milk to room temperature. Whisk together the flour, baking powder, and salt.
• Beat together the mayonnaise and sugar in a large bowl until they are well combined. Add half the flour mixture, then beat in the milk. Beat in the remaining flour mixture. (The batter will be loose.) Divide the batter evenly among the cupcake liners.
• Bake until the cupcakes are puffed and a toothpick inserted into the center comes out clean, about 20 minutes. Remove from the cupcake cups and cool on a rack.

Make the icing:

• Melt the butter in a medium saucepan over low heat. Add the chocolate and stir until it is melted. Remove from the heat and add the confectioners’ sugar, milk, vanilla, and salt and beat with a wooden spoon until the icing is smooth and glossy. Let the icing cool slightly.
• Spread the icing over the cooled cupcakes and serve.