Line a 20cm (8in) square tin with a double layer of foil so that it hangs over the sides of the tin. Melt 75g (3oz) butter. Tip crushed shortbread into a bowl and mix in melted butter. Cool for 10 minutes, then press evenly over the base of the tin. Chill for 30 minutes. Break white chocolate into a heatproof bowl and melt over a saucepan of barely simmering water. Remove from water and stir in caramel and flavouring to taste, if using. Spread over shortbread base. Leave to cool, then chill for 30 minutes to firm up. Break dark and milk chocolate into a saucepan. Add remaining butter along with 3 tablespoons water. Heat gently, stirring, until melted together. Remove from the heat, stir well and leave to cool for 15 minutes, then spoon on top of caramel filling andsmooth top. Scatter with the crushed eggs; leave to cool completely, then chill for at least 1 hour to set. To serve, carefully remove from the tin using the foil to help you and place on a board. Cut into 12-15 bars. Keep chilled.