Line two 20cm round cake tins with baking paper.
Preheat your fan-forced oven to 160°C
Make one cup of espresso, sift the cocoa into a small bowl. Slowly pour the coffee into the cocoa powder and whisk until smooth. Refer to Note 1 for Espresso alternatives.
Sift flour, baking powder and bi-carb soda into a large mixing bowl, add the sugar and mix at a low speed to combine.
Add the buttermilk, oil, sour cream, and vanilla extract, then whisk on low speed until well combined.
Add eggs and cocoa mixture, mix on low speed until just combined. Scrap the sides and bottom of the bowl with a spatula and then whisk on medium-high until all combined.
Add cake mixture into the two cake tins as evenly as possible and place in the oven. Both on the middle rack not touching. Refer to Note 2 for tin positions for a small oven. Place a baking tray at the bottom of your oven to catch any drips.
Bake for 40 minutes or until the skewers come out clean.
Cool on a cooling rack in the tins for 15 minutes. Remove from the cake tins, and place the cakes top side down. Allow cakes to cool completely.