Make the custard first as it needs to chill. Put all the ingredients, except the vanilla, in a large pan and bring gently to the boil, whisking all the time, until the chocolate has melted and you have a silky, thick custard. It will take 5-7 mins from cold. Stir in the vanilla and a generous pinch of salt, then scrape the custard into a wide, shallow bowl. Cover the surface with cling film, cool, then chill for at least 3 hrs or until cold and set.
Heat oven to 180C/160C fan/gas 4. Grease then line the bases of 2 x 20cm sandwich tins. Melt the butter in a pan, then remove from the heat and beat in the oil, buttermilk, coffee and eggs. In a large bowl, whisk the dry ingredients togetherplus 1/4 tsp salt (saves sifting) and squish any resistant lumps of sugar with your fingers. Tip in the wet ingredients and whisk until smooth.
Divide the batter between the prepared tins and bake for 25-30 mins until risen and a skewer inserted into the middle of the cakes comes out clean. Cool for 10 mins, then transfer to a rack to cool completely, parchment-side down.
Remove the parchment linings from the cakes. If the cakes are domed, trim them flat. Now cut each cake across the middle using a large serrated knife. Put your least successful layer and any trimmings into a processor and pulse it to crumbs. Tip into a large bowl.
Sit one layer on a cake plate and spread it with a quarter of the custard. Sandwich the next layer on top, add another quarter of the custard, then top with the final layer of cake. Spoon the remaining custard on top of the cake, then spread it around the top and down the sides until smooth. Chill for 15 mins to firm up the custard again.
Hold the cake over the bowl containing the crumbs, then sprinkle and gently press a layer of crumbs all over the cake. Brush any excess from the plate. You’ll have some crumbs left. Chill for 2 hrs, or longer, before serving, and eat it cold. Can be made up to 2 days ahead. The cake gets fudgier and more enticing the longer you leave it.