Preheat the oven to 180C/350F and line the base of a baking tin with greaseproof paper.
Place the eggs, sugars, buttermilk and sunflower oil into a bowl and combine with a whisk until the eggs are broken up and smooth.
In a small separate bowl, place the coffee, boiling water and cocoa powder and whisk until smooth.
Pour into the buttermilk mixture and whisk to combine.
In a separate bowl, place the flour, baking powder, baking soda and salt and stir to combine with a fork (this will help remove large lumps).
Add the dry ingredients into the wet ingredients and combine until your batter is smooth.
Pour into your cake tin and bake for 30 minutes or until a skewer comes out clean.
Remove and allow to cool before removing from the tin.
Turn onto a cooling rack to cool completely.