Preheat the oven to 150°C (conventional, not fan). Grease a 9 x 13 x 2 inch cake tin with Stork (or softened butter) and line completely with 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.
Pour the batter into the prepared tin. It will be pretty full, but it shouldn't overflow when baking. Firmly tap the tin down on the work surface a few times to help release any large air bubbles, then drag a blunt knife back and forth through the mixture a few times. This helps to prevent 'craters' from forming in the cake when it is baked (but don't worry too much— no one will notice once it's filled with icing anyway!).
Bake the cake in the centre of the preheated oven for 90-95 minutes, until a skewer inserted into the centre of the cake comes out clean, or with just a few moist crumbs. The cake will be very domed and the surface will be quite cracked— don't worry, that's how it's supposed to be!
Leave the cake to cool in the tin for about an hour or so, then run a palette knife around the edge of the tin to loosen it before turning it out onto a wire rack. Allow the cake to cool completely before assembling (or wrapping tightly in clingfilm and storing)— this is a large, dense cake so it will take a long time to cool (like... 3-4 hours!).