Preheat the oven to 180C/350F/Gas 4. Generously butter 4 pudding basins or ramekins (using 1oz/25g)
Melt the chocolate in a heatproof bowl set over a pan of simmering water. Remove from the heat and whisk in the remaining 75 g/3 oz of the butter until melted. Set aside.
Place the egg whites in a bowl and whisk until stiff peaks have formed. Whisk in half of the sugar, a third at a time, whisking well after each addition until stiff and very shiny.
In a separate bowl, beat the remaining sugar and egg yolks until pale and fluffy, then beat into the cooled chocolate mixture. Fold in the sieved plain flour. Fold in the meringue and use to fill the prepared individual basins two-thirds full. Arrange on a baking tray and bake for 12 minutes until well risen but still with a slight wobble in the middle. To serve, leave to settle for a few minutes and carefully invert on your serving plate.