Line the base of a 20 cm (8 inch) springform cake tin with nonstick baking paper. Melt the chocolate and butter in a heatproof bowl set over a saucepan of gently simmering water, stirring occasionally, making sure the water doesn't touch the base of the bowl. Meanwhile, whisk the egg yolks with the sugar in a bowl with a hand-held electric whisk until pale and thick. Stir in the melted chocolate mix.
Whisk the egg whites in a separate large, grease-free bowl until they form soft peaks. Fold a couple of tablespoons of the egg white into the chocolate mixture to loosen, then fold in the remaining egg white with a metal spoon. Pour the mixture into the prepared tin and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes. Remove from the oven, cover with foil (to prevent a crust forming) and leave to cool. Chill in the refrigerator for at least 4 hours or overnight.
Make the syrup. Combine all the syrup ingredients in a small saucepan and heat over a low heat, stirring, until the sugar has dissolved. Bring to the boil, then simmer for 10 minutes until syrupy. Leave to cool. Remove the cake from the refrigerator 30 minutes before serving in slices, with the syrup poured over.