Preheat the oven to 180°C (conventional, not fan). Grease two 8-inch round sandwich tins and line each with a circle of baking paper. Set aside.
Place the instant coffee granules into a medium bowl. Pour over the boiling water and stir to dissolve. Add the cocoa powder and whisk until smooth.
Gradually add the diced butter to the hot cocoa mixture, whisking in each addition until all of the butter has been added and everything is smooth.
Sift the flour, caster sugar and bicarbonate of soda into a large bowl. Whisk together until well-combined, then set aside.
Add the buttermilk, oil and vanilla extract to the warm cocoa mixture. Whisk until incorporated.
The cocoa mixture should now be cool. Add the eggs and whisk thoroughly until completely incorporated.
Pour the cocoa mixture into the dry ingredients gradually, whisking all the time, until the batter is smooth and well-combined, with no lumps of flour.
Divide the batter evenly between the two tins. They will be pretty full, but they shouldn't overflow when baking. Firmly tap each tin down on the work surface a few times to help release any large air bubbles. This helps to prevent 'craters' from forming in the cakes when they are baked (but don't worry too much— no one will notice them once they're filled with icing anyway!).
Bake the cakes in the centre of the preheated oven for 50-55 minutes, until a skewer inserted into the centre of the cake comes out clean, or with just a few moist crumbs. The cakes will be very domed and the surface will be quite cracked— don't worry, that's how it's supposed to be!
Leave the cakes to cool in the tins for 20 minutes or so, then run a palette knife around the edge of the tins to loosen them before turning them out onto a wire rack. Allow the cakes to cool completely before assembling (or wrapping tightly in clingfilm and storing).