Preheat the oven to 160°C fan/180°C/350°F. Grease and line 2 (or 4 if you have them and the oven space!) 6" loose bottomed cake tins.
Start by making the base cake mixture. You can make this like you would a regular cake, by creaming the butter and sugar, then beating in the eggs, milk and vanilla and then folding in the flour and raising agents. However, as you're going to be splitting the mixture and adding more flavourings, it really is easiest to simply make this in the food processor. The results are just as good.
To prepare the cake mixture in the food processor: place the butter, sugar, flour, baking powder and bicarb into the bowl of the food processor. Pulse a few times until the ingredients are combined. Add the eggs one by one, processing briefly after each one. Finish by adding the milk and vanilla seeds and process until just combined. Take care not to overmix the batter.
The next stage requires just a little bit of maths if you'd like to be accurate, but don't worry if you don't have scales or don't want to do this. Weigh the entire batter mix. You then want to separate the batter into 3 portions - half and two quarters. Into the largest portion, carefully fold in the espresso powder. Into one of the smaller portions, fold in the cocoa powder. Leave the remaining smaller portion as it is.
Split the coffee mixture between the two tins and level the tops. Place in the oven and bake for 20 minutes or until a skewer inserted comes out clean. Leave to cool for 5 minutes in the tin before removing and transferring to a cooling rack. Clean the tins, grease and line again and pour the vanilla and chocolate portions of the batter into each one. Bake as above.