Heat oven to 160C/fan 140C/gas 4. Butter, double-line and wrap the sides of the 30cm cake tin as before. Put the butter and chocolate into a medium saucepan, then stir over a low heat until melted and smooth. Stir in the coffee and vanilla.
Sift the flour, baking powder and bicarbonate of soda into the biggest bowl you have. Add the sugar, breaking down any lumps with your fingertips if necessary. Beat the eggs and soured cream together in a jug or bowl and pour into the flour mix. Pour in the melted chocolate mix as well, then stir with a wooden spoon until you have a thick, even chocolaty batter.
Pour into the prepared tin and bake for 2½ hrs - don’t open the oven door before 2 hrs is up, as this will cause the cake to sink. Once cooked, leave in the tin to cool completely. The unfilled cake will keep for up to four days, wrapped as before, or frozen for a month.