Preheat the oven to 350°F (180°C), or 160°C for a fan oven.
Prep 2 x 8-inch/20cm cake pans by greasing them with butter and lining them with parchment paper.
In a bowl sift the dry ingredients (flour, cocoa powder, baking powder, baking soda, and salt), add the sugar, whisk and set aside.
In a separate bowl, combine the eggs with the buttermilk or milk, oil, and vanilla extract. Mix well.
Add the wet ingredients to the dry ingredients and mix to just combine (do not overmix, you don't want to work the gluten for a moist cake).
Stir in boiling water (mixed with espresso powder, if using) this will thin out the batter.
Pour the batter into the cake pans, and bake for 30 minutes or until a toothpick inserted in the center of the cake comes out clean.
Remove the cakes from the oven and allow to cool in cake pans for 10 minutes. Gently turn out on a cool rack upside down, and leave them to cool completely.