Preheat oven to 180°C/350°F/ Gas Mark 4. Grease moulds or paper cups. Melt chocolate in a double boiler and add five tablespoons of boiling water. Cool this mixture slightly. Beat butter until soft. Add the sugar gradually.
Blend these ingredients until they are very light and creamy. Beat in one egg yolk at a time. Add the chocolate mixture and mix. Sieve the flour with baking powder and salt. Mix milk and vanilla essence.
Add the flour mixture in three parts to the butter-sugar mixture alternately with milk. Beat the batter until it is smooth after each addition.
Whip egg whites until stiff but not dry. Fold them lightly into the cake batter. Pour into prepared moulds or paper cups till two thirds full and bake at 180°C/ 350°F/ Gas Mark 4 for thirty minutes.
Cool on a wire rack before de-moulding. Sprinkle with icing sugar and serve.