Preheat the oven to 160°C. Line a 7.5-inch cake tin with parchment paper.
In a bowl, mix together all the dry ingredients: flour, blended walnuts, bicarbonate of soda, baking powder, salt, cinnamon, mixed spice, and ground ginger.
In a separate large bowl, whisk the caster sugar, soft dark brown sugar, and eggs until light and fluffy.
Add the vegetable oil, vanilla extract, grated carrots, and orange zest to the sugar mixture and combine well.
Pour the dry ingredients into the wet mixture and stir until just combined.
Pour the cake batter into the prepared tin and bake for 1 hour, or until a skewer inserted comes out clean.
Allow the cake to cool completely before adding the ganache.
Melt the white chocolate in a bain-marie (double boiler). Once melted, remove from heat and stir in the double cream.
Add the Philadelphia cream cheese and whisk the mixture until stiff and smooth.
Smooth the ganache onto the cooled cake. For crisp edges, leave the cake in the tin while spreading the ganache.
Place the cake in the fridge for 2 hours to allow the ganache to set.