Preheat oven to 160°C fan. Grease a Bundt tin thoroughly and dust with cocoa powder.
Melt the dark chocolate, milk chocolate and butter together over a low heat. Whisk until smooth.
Split the vanilla pods and scrape out the seeds. Whisk the eggs, sour cream, oil and coffee together, then add the vanilla seeds.
Whisk in both sugars, followed by the melted chocolate mixture.
Sieve the remaining cocoa powder, flour, bicarbonate of soda and salt into a bowl. Fold into the wet ingredients until smooth.
Pour into the prepared tin and bake for 40–45 minutes, until a skewer inserted into the centre comes out clean.
Leave in the tin for 10 minutes, then turn out onto a wire rack and cool completely.
Whisk the egg yolks, sugar and cornflour together in a bowl.
Heat the milk and Ovaltine in a saucepan until steaming and the Ovaltine has dissolved.
Gradually pour the hot milk mixture over the egg yolks, whisking continuously.
Return to the pan and cook over a low heat, whisking constantly, until thickened.
Pour over the chopped chocolates and stir until melted and glossy.
Pour some sauce over the cooled Bundt cake and leave to set for 10 minutes. Serve the remaining sauce warm on the side.
