Prepare two 8-inch cake pans by spraying with baking spray and lining the bottom with baking parchment
Sift the cocoa powder into the bowl of a stand mixer. Add the flour, sugar, baking powder, baking soda, and salt. Using the paddle attachment, stir through flour mixture until ‘sifted’ and well combined.
Measure 250ml milk out into a large jug. Add the eggs and whisk lightly to break up the yolks. Add the vegetable oil, vanilla and vinegar, and whisk again to combine. With the stand mixer running on low speed, pour the milk mixture into the dry ingredients in a steady stream.
Once the jug is empty, measure 125ml strong black coffee into it. Add this to the cake batter, and stir again.
Scrape down the sides of the bowl and beat on high speed for another 30 seconds.
Distribute cake batter evenly between the two prepared cake pans - about 720g in each.
Bake for 30-45 minutes, until the top of the cake feels firm to the touch.
Remove from the oven and set the pan on a cooling rack.
Cool for 5 minutes in the tin, then invert the cakes and peel off the baking parchment.
Leave to cool completely.
Chocolate buttercream
Place the egg whites and sugar into a heatproof bowl, and set over a bain-marie.
I use the bowl of a stand mixer, to make everything cleaner and easier.
Stir gently over a medium heat until the sugar has all dissolved.
Whisk the meringue mixture for about 7 minutes, until cool and thick.
Add the butter, a little at a time, and continue whisking until the buttercream is smooth.
Melt the dark chocolate in the microwave, in short bursts, stirring regularly.
Allow the chocolate to cool a little, then add to the buttercream and whisk again.
Assembly
Trim the tops of the cooled cakes, using a bread knife.
Split the cakes in two, to make 4 thin cakes.
Stack the cakes, spreading a layer of the chocolate buttercream between each one.
Spread the remaining buttercream all over the top and sides.
Create an attractive swirl pattern in the buttercream, if you like.