Adjust rack to middle position and preheat oven at 170℃ / 340℉;
Grease the base and sides of a 20cm/ 8-inch baking pan with butter, then line with parchment paper on the base only;
In a bowl, combine cocoa powder, instant coffee powder and hot water, set aside for 5 minutes to allow it to bloom - this will accentuate the rich, chocolatey flavor;
In a separate bowl, mix the granulated sugar and brown sugar. Sift in the plain flour, cornflour, baking powder and baking soda. Add salt and whisk everything together until well combined.
To the bowl with the bloomed cocoa powder, add the remaining wet ingredients, i.e. vegetable oil, apple cider vinegar, vanilla, sour cream and an egg. Whisk until just combined;
Poor the wet ingredients into the bowl of dry ingredients and whisk together until just combined;
Pour the batter into the cake tin and bake at 170℃ / 340℉ conventional (top and bottom heating i.e. no fan) for 28-30 minutes;
Perform a skewer test to ensure it comes out clean, then remove the cake from the oven.
Allow the cake to cool completely in the cake pan, or for at least 30 minutes;
Run a knife around the side of the cake pan and release the cake;
Transfer cake to a stand, then frost using an offset spatula or spoon, ready to serve ENJOY!