For the chocolate sponge: Preheat the oven to 180 degrees or 160 if using a fan oven. Grease three x 20cm sandwich cake tins and line the base of each with a disc of parchment paper.
Sift the cocoa powder into a bowl and add 135ml boiling water to make a paste. Whisk until smooth and set aside to cool slightly.
Cream the butter and sugar together using an electric beater, stand mixer, or by hand, until pale, creamy and soft. Add the eggs one by one, letting each incorporate fully into the mixture before adding the next.
Scrape the cocoa paste into the butter, sugar and egg mix, continuing to beat until fully combined. Sift the flour into the mix and add the salt and mix on a low speed until the flour is incorporated.
Divide the mix between the three cake tins and bake in the preheated oven for 25 minutes, or until a skewer inserted into the centre comes out clean.
Remove the cakes from the oven and leave to cool slightly in the tins, before turning out on to a wire rack and leaving to cool fully.
For the chocolate buttercream: Break the chocolate into small pieces and place in a bowl over a pan of simmering water, stirring occasionally, ensuring that the bottom of the bowl doesn’t touch the water. Alternatively, melt the chocolate in a heatproof bowl in a microwave for 30 seconds, giving the chocolate a little mix, repeating until the chocolate is fully melted. Remove the chocolate from the heat and allow it to cool slightly.
Beat the butter with an electric beater, stand mixer, or by hand, until pale and creamy. Sift the icing sugar into the bowl, add the salt, and continue to beat until fully incorporated.
Scrape the chocolate into the buttercream and mix well until fully combined. Loosen the buttercream with the milk, adding a tablespoon at a time until it is silky.
To assemble: Spread a quarter of the buttercream onto one of the chocolate sponges and place the second sponge on top. Repeat with another quarter of the buttercream and place the remaining sponge on top. Cover the cake with the remaining buttercream, and store in the fridge until ready to serve, or serve immediately.