Grease and line a 20cm x 20cm brownie tin. Heat oven to 180C/160C fan/gas 4. Put the butter and both types of chocolate in a heat proof bowl and either melt in the microwave (in 30 second bursts, stirring after each) or set over a pan of barely simmering water, stirring every now and then until the chocolate has melted.
Leave the chocolate and butter mixture to cool a little while you whisk the eggs and caster sugar in a large bowl using electric beaters. Once the mixture is pale, fluffy and looks like it’s roughly doubled in volume, whisk in the melted chocolate. Fold in the flour, cocoa powder and mixed spice until no pockets of flour remain then pour into your prepared tin. Level the top with a spatula and bake for 20-25 mins. The top should look set and shiny but should be a little wobbly if you gently jostle the tin.
Leave the brownie to cool completely in the tin then chill in the fridge until set. While the brownie cools brush the rosemary sprigs and glacé cherries with egg white, dab off the excess with kitchen paper then dredge in caster sugar until well coated. Leave to dry on a wire rack. Put the chocolate truffles onto a sheet of baking paper or foil then spray with the edible gold lustre.
Dust the chilled brownie with icing sugar to create a snowy surface and top with amaretti biscuit pieces then poke the crystalised rosemary sprigs into the surface at random intervals (cut the brownie into pieces and dust on icing sugar first, if you like). Nestle a glacé cherry or gold truffle alongside the rosemary sprig then add the buttons and silver balls.