Pour in 40ml just-boiled water (you can use the water from the pan used to melt the chocolate) and mix. Pour the wet ingredients and melted chocolate into the dry, then stir to combine. In a jug, whisk the 2 tbsp oil and the egg until combined, then stir in the soured cream. In a large bowl, combine the flour, cocoa, sugar, baking powder and a pinch of salt. Melt the chocolate in a heatproof bowl over a pan of just simmering water (don’t let the bowl touch the water), then leave to cool slightly. For the cake, heat the oven to 150☌ fan/gas 3½ and grease and line the cake tin base with oil and baking paper.Leave to cool, cover with baking paper touching the surface and chill for at least 2 hours. Leave for 1 minute to melt, then stir to make a chocolate custard. Put the chopped chocolate in a heatproof bowl, then pour the custard over. To check, dunk the spatula in the custard then push a finger across it – if the line you’ve created holds for a few seconds it’s ready. Cook gently until the custard thickens enough to coat the back of the spatula (about 5 minutes). Use a rubber spatula to gently stir the mixt as it heats through (you don’t want to add bubbles of air). Pour the mixture back into the pan and put over a low-medium heat.Once the milk is steaming hot and quivering (but not quite boiling), gradually pour it into the egg mixture, whisking constantly until combined. In a large bowl, use a balloon whisk to beat the egg yolks and sugar together until combined and a little paler. Put over a medium heat and warm through, stirring every now and then to stop it catching.

For the custard, pour the milk and cream into a pan.

Black forest trifle cake