Whisk together the butter and sugar for the cake in a bowl using a hand held mixer.
Sift the cocoa, flour and salt together.
Whisk one egg and a third of the flour mixture into the butter and sugar.
Repeat this twice until your batter is smooth and thick.
Pour the batter into the two tins and bake for 25 minutes until the cakes become springy.
After 25 minutes, take the cakes out of the oven and allow them to sit in the pan for 10 minutes before turning them onto a wire rack and letting them cool.
Make the buttercream y melting the chocolate in a microwave and allow it to cool, stirring so it stays liquid.
Mix half the icing sugar into the butter and then stir in the sugar and chocolate.
Place the buttercream in a fridge for half an hour.
Spread the butter cream over the base of both cakes. Press the two cakes together gently.