Preheat oven to 180°C. Grease and line the bases only of two 20cm springform cake pans with baking paper.
Using an electric mixer, beat butter and sugar until thick and pale. Add eggs, 1 at a time, beating well after addition. In a separate bowl, whisk stout and cocoa until combined. Sift flour, baking powder and bicarb soda together. Beat one-quarter flour mixture into butter, then one-quarter stout mixture. Repeat, alternating between the flour and stout mixtures, beating constantly until combined. Weigh batter and divide by 5 (this is the amount of batter per cake pan – approximately 580g per pan). Pour one portion batter into each pan, reserving remaining batter. Bake for 30 minutes or until a skewer inserted into the centre comes out clean. Cool for 10 minutes in pans, then turn out onto a wire rack to cool completely. Clean pans, then grease and line the base. Fill each pan with approximately 580g batter and repeat baking process. Repeat process with one cake pan and remaining batter.
For the butterscotch sauce, cook sugar in a pan over medium heat, swirling pan, until caramelised. Add butter to the pan and swirl. Remove from heat and add cream, swirling pan until smooth. Cool.
To make caramel chocolate shards, melt chocolate in a heatproof bowl set over a pan of simmering water (don’t let bowl touch water). Pour melted chocolate onto a baking paper-lined baking tray, smooth out, then roll up baking paper and tie with kitchen string. Chill for 15 minutes, then unroll paper, breaking shards of chocolate.
For the buttercream, beat butter using electric beaters until pale. Gradually add icing sugar, beating constantly, then beat in milk. Add chocolate and beat until smooth.
To assemble cake, spread one-quarter icing over one cake. Repeat layering three more times, finishing with a final layer of cake. Pour over butterscotch sauce and top with caramel chocolate shards to serve.