Place the butter and sugar in a large, microwave safe bowl and melt in the microwave on 50% power in 30 second bursts (stirring at each interval) until the sugar has almost dissolved and the butter has melted.
In a bowl, whisk the sifted icing sugar, softened butter and lemon juice until creamy. If the frosting is too thick to spread, add a little more lemon juice. Spread on top and chill.