Preheat the oven: Preheat oven to 180ºC (350ºF) fan-forced. Line a square 9-inch x 9-inch (22cm x 22cm) baking tin with parchment paper and set aside. Alternatively, if using a non-stick pan, grease it with melted butter, line just the base with parchment, and dust the sides with a light layer of flour, tapping out the excess. This ensures smooth sides for the finished cake.
Prepare raisins: In a medium-sized pot, add raisins (or sultanas if using) and water. Place over high heat and bring to a boil.
Boil raisins: Boil for 1 minute, stirring occasionally with a wooden spoon, then remove from the heat.
Add butter: Add cubed butter to the raisins and stir until completely melted.
Cool the mixture: Pour the mixture into a large clean mixing bowl and allow it to cool for 20 minutes, just until it’s not too warm. (You want to avoid curdling the eggs when they are added.)
Combine dry ingredients: In a medium-sized mixing bowl add the flour, baking soda, cinnamon, nutmeg, allspice and the salt. Whisk until well combined and set aside.
Mix wet ingredients: Add sugar and beaten eggs to the cooled mixture. Mix until well combined, about 30 seconds.
Add the flour mixture: Add the prepared flour mixture to the wet ingredients and whisk until combined and no pockets of flour remain. Take care not to over-mix.
Add walnuts: Fold through the chopped walnuts.
Pour batter into pan: Pour the batter into the prepared pan, spreading it out evenly.
Bake: Bake for 35-40 minutes or until a toothpick inserted into the middle comes out clean or with a few moist crumbs. The cake should also spring back when lightly pressed in the middle.
Cool the cake: Remove from the oven and cool in the pan for 15 minutes.
Transfer and cool completely: Transfer the cake to a wire rack to cool completely.
Finish & Serve: Dust with powdered sugar, if desired. Enjoy!