Grease and flour one 9"x2" round pan and line the bottom with parchment paper.  Preheat the oven to 350F.

Combine in a heatproof bowl placed over hot water the butter and chocolates.  Stir until melted, then cool 5 to 10 minutes.

In the bowl of an electric mixer at high speed, use a whisk attachment to beat the eggs, 1/2 cup sugar, and vanilla.  Continue beating until triple in volume and the batter forms a ribbon when the beaters are lifted. 

Meanwhile, in a saucepan, heat to a full boil 1/2 cup sugar and the corn syrup (or golden syrup). Remove from the heat and slowly beat the hot syrup into the eggs with the mixer at low speed.  Beat at medium speed until the batter reaches the top of the bowl, about 8 minutes, then decrease the speed to low and beat in half of the chocolate mixture.  Before it is fully incorporated, take off the whisk attachment and fold in the remaining chocolate by hand.

Pour the batter into the pan.  Place the pan in a larger pan and fill the larger pan halfway with hot water.  Bake for 40 minutes.  The cake will look set.  Cool the cake on a wire rack for 15-20 minutes, then turn it out of the pan onto a cake round.  Cool completely, then chill before glazing.

Serve the cake at room temperature.