1.Preheat oven to 180°C/350°F. Grease two deep 20cm (8in) round cake pans; line bases with baking paper.
2.Beat butter and sugar in a small bowl with an electric mixer until light and fluffy. Beat in eggs one at a time. Transfer mixture to a large bowl; fold in sifted flours, soda and cocoa with combined coffee, colouring and buttermilk, in two batches. Divide mixture into pans; smooth the surface.
3.Bake cakes for 45 minutes or until a skewer inserted into the centre comes out clean. Turn cakes, top-side up, onto wire racks to cool.
4.Make rich chocolate frosting. Place chocolate and butter in a small heatproof bowl over a small saucepan of simmering water (make sure the base of the bowl doesn’t touch the water); stir until smooth. Remove from heat. Cool at room temperature until spreadable, stirring occasionally while cooling.
5.Stir mascarpone and sifted icing sugar in a medium bowl until combined.
6.Place one cake on a plate or cake stand; spread with mascarpone mixture. Top with second cake; spread with rich chocolate frosting. Dust with extra cocoa.
The cakes can be made up to 2 days ahead; store in airtight containers at room temperature. Assemble and decorate on the day of serving.