1. Preheat the oven to 180C/350F/Gas 4.
2. To make the sponge, mix together the butter and sugar in a food processor until pale and fluffy.
3. Melt the 100g chocolate bar in a bowl over a saucepan of boiling water. Only a little boiling water is enough to melt the chocolate, about half an inch/1.27cm. Mix one Tbsp of Grand Marnier in the melted chocolate.
4. Pour the butter and sugar mixture into a bowl, then gradually add the eggs, the sieved flour, the tbsp of cocoa powder, and the melted chocolate until just combined. Add a tsp of Valencia orange essence to the mix.
5. Grease a small cake tin with a loose base. Pour the sponge mixture in. Transfer to the oven to bake for 30 minutes or until risen and cooked through.
6. While the chocolate sponge is in the oven, prepare the oranges. Melt 25g of butter in a frying pan. Add the orange slices, the tbsp of caster sugar, wait until the sugar has dissolved then pour 1 tbsp of Grand Marnier. Caramelise for a minute before transferring the oranges and caramelised butter mix into a baking tin to roast for 10 minutes or until the oranges are golden -brown.
7. Prepare the double cream by whisking it a little and adding to it some orange zest.
8. Once cool turn the sponge on a plate or serving dish. Decorate the top with the roasted oranges. The chocolate sponge can be flambéed at the table for maximum effect with another tbsp of Grand Marnier. Serve with a dollop of the zesty double cream.