Grandma's Date-Nut Bread
1 cup chopped pitted dates 1 cup chopped walnuts 1 cup sugar 1 egg, well-beaten 1 teaspoon baking soda 1 teaspoon vanilla extract 1 ½ cups boiling water 1 ¾ cups all-purpose flour 2 teaspoons butter, or as needed 7 pitted prunes, chopped ¼ cup unsalted butter, softened ½ cup raisins ½ teaspoon salt