Chocolate can be a tough match with wine, owing to its tannins and bitter flavours. For bitter chocolate consider fortified wines or liqueurs.
Milk Chocolate - Light, fresh, sweet wines work here.
Dark Chocolate - Fortified wines such as a late LBV ports, or rich reds from the South West of France or northern Italy.
Chocolate Cake - Orange flavoured muscats are good here, but also some liqueurs.