Preheat oven to 160°C fan forced. Grease and line a 3cm-deep, 20cm x 30cm (base) lamington tin.
To make the bottom layer, combine the chocolate and butter in a heatproof bowl and set over a saucepan of simmering water, stirring until just melted. Add the eggs and sugar and beat until smooth. Sift in the flour and cocoa and fold through with the macadamias until well combined. Spoon into the prepared tin and bake for 20-25 minutes or until just set. Allow to cool in the tin.
To make the middle layer, combine the macadamia butter, icing sugar, honey and milk in a medium bowl and whisk until smooth. Poor over the cooled chocolate slice and refrigerate for 2 hours or until set.
To make the icing, melt the chocolate gently in a bowl over simmering water and spread over the macadamia and honey layer. Sprinkle with finely chopped macadamias and refrigerate for an hour. To serve cut into bars.