Preheat the oven to 180C/350F/Gas 4.
Grease the insides of the dariole moulds with butter, and then coat with cocoa powder, discarding any excess powder.
Put the chocolate pieces and the butter into a bowl. Bring half a saucepanful of water to the boil, then place the bowl in the pan so that it is suspended over the water (do not allow the water to touch the base of the bowl). Stir the chocolate and butter until they have melted into a smooth liquid.
In a separate bowl, whisk the sugar and the egg together, then whisk the hot chocolate liquid into the cold sugar and egg.
Fold the flour into the chocolate mixture, then spoon the mixture into the dariole moulds.
Press the chopped strawberries into the centre of each chocolate fondant, and give each dariole mould a tap to get rid of excess air. Level off the mixture with a palette knife. Place on a baking tray and bake for 6-7 minutes.
To serve, using a palette knife, loosen the fondants from the moulds and turn them out onto a serving plate. Put the icing sugar into a sieve and dust it over the fondants. In a separate bowl, mix the Greek-style yoghurt with the lime zest and serve as a side dish.