Step 1
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.
Step 2
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.
Step 3
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.
Step 4
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.
Step 5
Cool on a wire rack before de-moulding. Sprinkle with icing sugar and serve.