Pre heat the oven to 180°c.
Grease and line a 20cm x 20cm traybake tin.
Melt the butter and sugar in a large saucepan over a low heat, stirring well to ensure that the sugar dissolves.
Remove from the heat and allow to cool slightly.
Add the orange zest, mincemeat, cinnamon and beaten egg.
Stir in the flour and mix thoroughly.
Pour the mixture into the traybake tin and bake for 50-55 minutes, or until a skewer inserted into the centre comes out just slightly sticky.
Allow to cool in the tin.
For the buttercream: Break up the chocolate, add half to a bowl and place in the microwave in bursts of 20 seconds until melted.
Then add the rest of the chocolate and stir until completely melted. Allow to cool.
Place the butter and half of the icing sugar in a bowl and beat until smooth.
Add the remaining icing sugar and mix until smooth and creamy.
Add the vanilla extract and white chocolate and mix well.
Remove the traybake from the tin, top with the buttercream, then decorate with white chocolate shavings, chopped pistachios and mixed peel.
