Spiced Prune Snack Cake - California Prunes

Ingredients

• 1/4 cup small-diced prunes (about 7)
• 1 cup boiling water
• 6 tablespoons salted butter (melted)
• 1/2 cup packed dark brown sugar
• 2 egg yolks
• zest of 1 Navel orange (plus more for garnishing if desired)
• 1 teaspoon pure vanilla extract
• 1 cup gluten-free or conventional all-purpose flour
• 1/2 teaspoon ground ginger
• 1/2 teaspoon ground allspice
• 1/4 teaspoon coarse kosher salt
• 1/2 teaspoon baking powder
• 2 tablespoons chopped walnuts
• 1 tablespoon candied ginger pieces