Prepare a 1.1lt pudding basin by placing a small circle of greaseproof paper into the bottom of the basin and greasing well.
Melt the chocolate in a bain marie or a bowl over a simmering saucepan of water.
Once melted stir in the double cream and golden syrup and pour into the bottom of the pudding basin. Set aside whilst you prepare the sponge.
Cream the butter and sugar together until light and fluffy.
Add the eggs one at a time and then the vanilla extract.
Sift together the flours, cocoa powder, baking powder and salt then beat into the rest of the sponge mixture.
Pour the sponge mixture into the pudding basin on top of the chocolate cream and smooth the top.
Cut a large circle of greaseproof paper and tin foil to fit over the top of the basin with a very generous overlap. Lay the tin foil on top of the greaseproof paper then make a pleat in the centre of the two.
Place the greaseproof paper/foil lid over the top of the basin, greaseproof paper down, and use string to secure the lid, tying the string just underneath the lip of the basin. Cut away the excess paper just underneath the string.
Place a metal trivet or a folded up tea towel in the bottom of a large stockpot or saucepan and then place the pudding basin on top so that it doesn’t touch the bottom of the pot.
Fill the pot halfway up the sides of the pudding basin with boiling water then place the lid on the pot. Bring the water up to boil then turn down to simmer for 1½ hours. Check the water level every so often to make sure it doesn’t boil dry.
Once ready, remove the pudding basin carefully from the stockpot, remove the paper/foil lid and turn upside down onto a serving plate. Serve warm with plenty of custard.