Melt the chocolate in a heatproof bowl set over a pan of simmering water. Remove from the heat and whisk in the remaining 75g/3oz 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 meringue and use to fill the prepared cups two-thirds full. Arrange on a baking sheet and bake for 10-12 minutes until well risen but still with a slight wobble in the middle.
I like to serve the chocolate fondants in the tea cups on a saucer and then just drop a spoon of vanilla ice cream on top and a little pouring cream also if you wish to be very indulgent.