1. Preheat the oven to 180 ̊C, gas mark 4. Grease and line the base of 2 x 20cm sandwich tins with parchment. Warm the cream, vanilla and 150g brown sugar in a pan, stirring until the sugar has melted, then heat until just steaming. Add the chocolate and stir until melted; set aside.
2. In a separate bowl, using electric beaters, cream the butter and remaining 150g sugar until light and fluffy. Beat in the eggs, a little at a time, until incorporated, then mix in the melted chocolate mixture. Sift in the flour, salt and bicarbonate of soda and beat to a smooth batter. Divide between the 2 tins and bake for 30 minutes or until a skewer inserted into the centre of the cakes comes out clean; cool in the tins on a wire rack.
3. For the buttercream, melt the chocolate in a microwave or a bowl set over a pan of barely simmering water; cool for 10 minutes. In a separate bowl, cream the butter and icing sugar for 2 minutes until smooth, then beat in the cocoa and a pinch of salt (optional). Stir through the cooled, melted chocolate; if the icing is too soft, chill for 10 minutes to firm.
4. Remove both cakes from the tins and peel away the parchment. Place one cake on a plate or cake stand and spread the top with a layer of icing. Sit the second cake on top and spread all over with the icing, covering both sponges. Scatter grated chocolate on top, if liked.