Butter 4 ramekins or individual pudding basins and chill.
Melt the chocolate and butter in a heatproof bowl over a pan of simmering (not boiling) water.
Allow to cool slightly.
Whisk the eggs and sugar for 8-10 minutes, until thick and creamy.
Gradually fold in the melted chocolate mixture.
Sift in the flour and stir until combined. Pour into the ramekins and freeze overnight.
Heat the oven to 190°C (170° fan) 375°F gas 5.
Bake the puddings from frozen for 12-14 minutes, until the middle puffs up slightly (do not open the oven while the puddings are baking).
The outer layer should be baked, but the centre should be soft.
Remove from the oven and cool for 2 minutes.
Put on serving plates and serve immediately.