Preheat the oven to 180°C. Grease and line a 20cm square cake pan with baking paper. Place the walnuts and sugar in a food processor and whiz until the walnuts are crushed. Add the melted dark and milk chocolates and pulse to combine.
With the motor running, add the eggs, 1 at a time, then add the spices, orange zest and a pinch of salt.
Pour the batter into the cake pan and bake for 35-40 minutes until just cooked through, but still soft in the center. Cool in pan for 20 minutes, then turn out onto a wire rack to cool completely.
For the icing, place the cream cheese in a food processor and whiz until smooth. Add cream, icing sugar and vanilla seeds, and whiz until smooth and combined.
Slice the brownie into 12 squares and top with cheese icing to serve.