1. Preheat the oven to 400°F (200°C). Generously butter the inside of six 6oz (175g) custard cups, and line each with a round of wax paper. Butter the paper.

2. Melt the chocolate in a heatproof bowl set over, but not in, a saucepan of simmering water. Remove from the heat.

3. Beat the butter and sugar in a bowl with an electric mixer on high speed for about 3 minutes, until light and fluffy. Beat in the eggs one at a time, beating well after each addition, then add the vanilla. Sift the flour and salt together and stir into the batter, then stir in the chocolate. Divide the batter among the cups. Place the cups on a baking sheet.

4. Bake for 12–15 minutes, or until the sides are set but the centers are still soft. Top each cup with a serving plate. Protecting your hands with a kitchen towel, invert both to unmold the cake. Remove the wax paper. Serve hot.