Begin by greasing a 10 × 7-inch baking dish with ½ cup (115g) of melted salted butter.
In a separate bowl, combine 1¾ cups (250g) of currants and golden raisins with 6 tbsps (75g) of granulated sugar, finely grated zest of 1 lemon, and ½ tsp pumpkin pie spice. This fruit mixture will be layered throughout the pudding to create pockets of sweetness and flavor.
Prepare about 8–12 thin slices of white bread by removing the crusts and cutting each slice into 3 strips.
Assemble the pudding in layers, starting with the bread. Immerse one side of the bread strips in the melted butter and arrange them buttered side down in the prepared dish.
Sprinkle half of the fruit mixture over this first layer, then add a second layer of bread strips placed buttered side up.
Sprinkle the remaining fruit mixture over the second layer, then finish with a third layer of bread strips, also buttered side up. This three-layer construction guarantees that every bite contains the proper balance of bread, fruit, and custard.
Whisk together 3 extra-large eggs, 1¼ cups (300ml) of milk, and ⅔ cup (150ml) of heavy cream until well combined. Pour this custard mixture evenly over the layered bread and fruit.
Sprinkle 2 tbsps of demerara sugar across the top for a crispy, caramelized finish. Allow the pudding to rest for 1 hour if time permits—this gives the bread time to absorb the custard and develop better texture.
Bake at 350°F for about 40 minutes, until the top is golden, crisp, and slightly puffed, indicating that the custard has set while maintaining its creamy interior.