Preheat oven to 350°F. Coat muffin pan with nonstick spray.
Combine flour, ginger, cinnamon, allspice, salt, baking powder, baking soda and black pepper in a medium bowl.
Pour ½ cup boiling water over butter and golden raisins in a large bowl, stir with a whisk until butter is melted. Whisk in sugar, molasses and egg. Add dry ingredients; whisk to blend. Stir in ½ cup walnuts using a rubber scraper. Scoop into prepared muffin pan.
Bake until a tester inserted into center of cake comes out clean, about 20 to 25 minutes. Let cool in pan for 10 minutes. Invert onto a wire rack; let cool completely. Combine confectioner’s sugar and water. Drizzle over muffins. Sprinkle with remaining 1 tbsp. walnuts.