In the bowl of a stand mixer, cream the butter, caster sugar and vanilla until light and creamy, about 5 minutes.
Place in the fridge for 10 minutes or so before frosting them if it's a hot day.
Assemble just prior to serving:
Beat together the mascarpone, cream, sugar and vanilla until just thick enough to dollop – be careful here as it’s a really fine line and if overbeaten the cream will be grainy and difficult to salvage. If this does happen, try adding more liquid whipping cream and stirring very gently until smooth.
Place one cake on your serving plate. Dollop with half the mascarpone cream and spread to the edges with an offset spatula or the back of a spoon. Carefully top with the second cake, and finish with the remaining cream. Use a sieve to dust with cocoa just before serving.