Preheat oven to 180°C / 160°C fan-forced. Grease a 20 cmx 30 cm lamington pan and line base and sides with baking paper, extending 3 cm over long sides

Stir the butter and chopped chocolate in a saucepan over low heat until mixture is smooth. Transfer to a medium bowl

Stir in sugar and eggs until combined. Stir in flour, sour cream and colouring until smooth and combined. Pour mixture into prepared pan. Bake, uncovered, for about 30 minutes. Cool completely in pan

Meanwhile, make the vanilla buttercream

Remove brownie from pan. Using a 6 cm star cutter, carefully cut 16 stars from brownie. Spread a layer of buttercream over half the brownie stars. Stack with remaining brownie stars to form 2 layers. Add a little dollop of buttercream to the top brownie star and scatter with crushed candy canes

Place butter and vanilla in a large bowl. Use a mixer to beat the butter mixture until pale. Gradually add sugar until smooth and combined. Adjust thickness of icing with milk