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. If using a non-stick tin, grease well with butter, line the base with parchment, and lightly flour the sides, tapping out excess flour. This ensures smooth cake edges.
In a medium sized pot, add raisins and water. Place over a high heat and bring to a boil.
Boil for 1 minute, stirring occasionally with a wooden spoon, then remove it from the heat.
Add cubed butter to the raisins and stir until completely melted.
Pour mixture into a large clean mixing bowl and allow mixture to cool for 15-20 minutes, just until it’s not too warm, you don’t want your eggs to curdle when you add it.
While it’s cooling down, roughly chop your walnuts into small pieces.
Add sugar and beaten eggs. Mix until well combined, about 30 seconds.
Sift over flour, baking soda, cinnamon, nutmeg and allspice. Add the salt then gently fold until incorporated, do not over-mix, just until no streaks of flour remain.
Fold through chopped walnuts.
Pour batter into prepared tin, spreading it out evenly.
Bake for 35-40 minutes or until 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.
Remove from oven and cool in the tin for 15 minutes.
Transfer to a wire rack to cool completely.
Optional: Dust with powdered sugar.
Cut into squares, serve and enjoy!