Heat oven to 350°F. Grease 13 x 9 x 2-inch baking pan. Bring water and molasses in saucepan to boiling. Stir in baking soda. Let cool.
Beat sugar, butter and eggs in bowl. In second bowl, combine flour, baking powder, cinnamon, ginger and cloves. Beat flour alternately with molasses, half at a time, into butter until well mixed. Pour into the prepared pan.
Combine flour, sugar, cinnamon, ginger, walnuts in small bowl. Cut in butter with pastry blender until mixture is coarsely crumbed. Sprinkle over gingerbread.
Bake in 350°F oven for 40 minutes.
Whisk sugar, milk and vanilla in small bowl until soft enough to drip from spoon. If needed, add more milk for proper consistency. Drizzle over top of cake.