Put raisins in a bowl. Pour boiling water over raisins; soak until raisins have flavored the water, about 20 minutes. Drain raisins, reserving the water. Discard raisins.
Preheat oven to 350 degrees F (175 degrees C). Grease a loaf pan with butter.
Bring reserved water to a boil in a small saucepan. Measure 3/4 cup boiling water into a bowl. Stir baking soda into water.
Put dates and prunes in a large bowl. Pour baking powder-water over dates and prunes; add unsalted butter and stir until melted. Stir sugar and vanilla extract into the fruit mixture; let cool to room temperature.
Stir egg into the fruit mixture to evenly coat. Mix flour and salt together in a small bowl; add to fruit mixture and stir. Fold walnuts into the mixture and pour into the prepared loaf pan.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 50 to 65 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack.
