Underneath its rich chocolate topping and layer of vanilla custard, this striking dessert from blogger Sasha Martin hides a light chocolate sponge cake soaked with orange blossom syrup.Slideshow: More Chocolate Cakes
In a saucepan, combine the milk, sugar and vanilla bean and seeds and bring to a simmer. Cook over moderate heat, stirring, until the sugar dissolves. In a blender, combine the flour and half of the hot milk mixture (but not the vanilla bean) and puree until smooth. Add the flour-milk mixture to the milk in the saucepan and cook over moderately low heat, whisking frequently, until thickened, about 8 minutes. Strain the custard into a medium bowl; discard the vanilla bean. Let cool until warm, then whisk in the butter a few pieces at a time. Press a piece of plastic wrap on the surface of the custard and chill for about 1 hour or until firm.
In a heatproof medium bowl set over a saucepan of simmering water, melt the chocolate, about 2 minutes. Remove the bowl from the pan and whisk in the oil. Let cool completely.
Using an offset spatula or the back of a spoon, spread the custard in an even layer all over the top of the cake. Carefully pour the chocolate on top of the custard and spread in an even layer. Chill until the chocolate sets, about 1 hour. Serve cold.
The cake can be refrigerated for 2 days.