Preheat an oven to 170°c/150°c fan (338°f/302°f).
Grease & line a 9" x 13" baking tin with baking parchment then set to one side.
Add the cocoa powder & instant coffee into a mixing bowl then pour over the boiling water & whisk to combine. Set this to one side, to let the cocoa bloom whilst we mix the rest of the batter.
Next, add the plain flour into a large mixing bowl with both of the sugars, baking powder, table salt & the bicarbonate of soda then whisk to combine.
Add the sour cream, vegetable oil, eggs into a jug, whisk to combine then add to the flour, along with the cocoa & water mix.
Whisk everything together until combined & smooth then immediately pour the batter into the lined baking tin. Bake in the preheated oven for 50 minutes - 1 hour, until a skewer inserted into the middle comes out clean.The bicarb will start to react as soon as the batter is mixed, so it's important to get it into the oven as soon as possible! As soon as the cake comes out of the oven, place the tin onto a wire cooling rack & let the cake cool completely, still in the tin. Once cool, cut the cake into 12 pieces.