Preheat your oven to 180°C/160°C Fan/350°F/Gas mar
Line and grease your baking tins. This cake can stick a little so I recommend lining both sides and base and then greasing the baking (parchment) paper as well.
Mix your instant coffee with your boiling water.
Mix in your cocoa powder until fully incorporated and no cocoa powder remains. I recommend using a fork or small whisk for this. Leave to cool until at room temperature.
Using your small saucepan, melt your butter over a low heat and also leave to cool.
Sift your flour, baking powder, bicarbonate of soda (baking soda) and salt together into your large bowl.
Using your balloon whisk (or a wooden spoon will be fine) stir in your sugar, until fully mixed in. Then make a well (a bit of a dent) in the centre.
Mix your melted butter, oil and vanilla essence into your coffee and cocoa mixture, again use a fork or small whisk to ensure it is really beaten in. Then pour into the well in the centre of your flour mixture.
Using your balloon whisk (or wooden spoon) mix it all together until no flour remains and it is all fully incorporated.
Next, mix in your sour cream, using a balloon whisk preferably or a wooden spoon. Once all mixed in stir in your lightly beaten egg, again ensure, fully mixed in with your balloon whisk.
Once the batter is fully mixed it will be quite runny and silky in texture.
Pour into your sandwich tins, ensuring equal amounts are in each (place your tins of weighing scales if you want to be totally accurate, for me this came to about 510 grams in each tin).
Bake in the centre of your oven for 30-35 minutes. The cake is done when it is pulling away slightly from the sides, it springs back when touched and when a skewer inserted into the centre comes out clean.
Remove and leave to cool in the tins for 5-10 minutes, run a knife around the edges, to ensure it has not stuck and then invert onto a cooling rack to cool completely before adding the frosting.