Put dark chocolate and milk in a small saucepan and melt over low heat.
Add egg and sugar to a bowl and mix well until combined.
Slowly pour the melted chocolate mixture into the egg mixture, stirring to combine.
Add sifted cake flour and gently mix until smooth.
Melt butter in a double boiler and add to the batter, mixing well.
Pour the batter into a mug or small bowl until it is about 2/3 full.
Microwave for about 1 minute and 30 seconds, or until risen, then remove.
Top with nuts or chocolate chips as desired and serve.