Heat oven to 180C/160C fan/gas 4. Line and grease a 20cm square brownie tin. Beat the butter and sugar using an electric whisk for about 5 mins or until the mixture is light and fluffy. Gradually beat in the eggs, one at a time, making sure each egg is well incorporated before adding the next. Add the mincemeat and orange zest, and beat again for a few secs to combine.
Sift in the flour, cinnamon and nutmeg, and stir with a large metal spoon until completely mixed. Tip into the tin and smooth using the back of the spoon. Bake in the oven for 30-35 mins or until a skewer inserted into the centre comes out clean.
Meanwhile, make the white chocolate icing. Put the chocolate in a large bowl. Pour the cream into a small saucepan and bring to the boil. Remove from the heat and pour over the chocolate. Stir until the chocolate has melted and it’s all combined, then set aside and leave to cool.
Leave the traybake to cool in the tin for 10 mins, then transfer to a wire rack. Once the cake is completely cool, spread the icing on top of the cake, then finely grate over the extra chocolate and cut into squares.