Grease & line an 8 inch round pan with baking parchment. Preheat the oven to 180 degrees C. (160 degrees for fan forced ovens)
Remove lumps if any from the soft brown sugar and add it along with the walnuts, coffee powder & mixed spice into a mixer jar and pulse the mixture for 2-3 second intervals till the walnuts break down and resemble fine bread crumbs. Do not use the 'grind' option of your mixie as the walnuts will begin to leave their natural oils and turn the mixture into a paste. Simply use the 'pulse' option of your mixie/food processor. Keep this mixture aside.
In a large bowl beat the butter & caster sugar till the mixture looks pale. Add the eggs, orange juice & zest and beat again.
Sift in the flour, baking powder & salt and beat till incorporated. Do not over beat the mixture. If you wish you can fold in the batter using a spatula, but the electric beater is fine too. This recipe is very forgiving.
Now using a spatula, spread out one third of the batter into the prepared tin. Sprinkle half of the coffee-walnut mixture over it. Spread another portion of the batter over it, followed by the remaining coffee-walnut mixture. Add the final portion of batter and even it out the surface.
Run a clean knife through in the shape of the number eight to achieve a marbled effect.
Bake in the preheated oven for 50-60 minutes or till the skewer inserted comes out clean.
Remove and leave the cake to cool in the tin for 10 minutes. Then invert it on a wire rack and allow it to cool completely. Cut and serve. This cake keeps well in an airtight container kept in a cool place for upto 3 days.