Grease and flour a 10-inch fluted tube pan; set pan aside. In a medium bowl, combine flour, baking powder, baking soda, salt and ginger; set aside.
In a large mixing bowl, beat butter with an electric mixer on medium to high speed for 30 seconds. Add sugar; beat until well combined. Add egg, egg yolk and maple syrup; beat for 1 minute more. Alternately add flour mixture and hot water to butter mixture, beating on low speed after each addition until combined. Spoon batter into the prepared pan; spread evenly.
Bake in a 375 degree F oven for 45 minutes or until a wooden toothpick inserted near the center comes out clean. Cool cake in pan on a wire rack for 10 minutes. Remove cake from pan. Cool completely.
In a medium bowl beat 1/2 cup powdered sugar and 2 tablespoons softened butter with an electric mixer on medium speed until combined. Beat in 1/4 cup pure maple syrup. Beat in 1 cup additional powdered sugar. Add 2 to 3 teaspoons milk, 1 teaspoon at a time, to make drizzling consistency. Makes about 1 cup.
Spoon Maple Icing over cake and sprinkle top with walnuts. Makes 12 servings.