Preheat your oven to 175°C / 350°F. Spray a 9 x 13inch baking tin with oil spray and line the bottom and sides with baking paper.Set aside.
Add the boiling water and instant coffee powder to a large heat proof jug and whisk until dissolved. Add the buttermilk and whisk.Set aside.
To a large mixing bowl add the flour, cocoa powder, baking powder, bicarb soda and salt and use a whisk to combine. Set aside.
To a separate large mixing bowl add the butter, sugar, brown sugar, and vanilla extract. Whisk using an electric hand mixer until pale and creamy, about 3 minutes. Add the eggs one at a time, mixing each time and scraping before each addition.
Add half the dry ingredients and half the wet ingredients and mix until well combined. Add the remaining wet and dry and repeat.
Pour the batter into the baking tin and spread using a spatula. Bake for 35 - 40 minutes. Once baked, allow to cool completely.