Preheat oven to 350° F.
Cream butter and sugar until light and fluffy. Beat in eggs and molasses. Combine flour, cocoa powder, baking powder and salt; blend into butter mixture along with the water. Stir in walnuts and cranberries. Divide dough in half. (Chill dough if necessary for ease of handling.)
On a lightly floured surface, shape each portion into two 10-inch long logs. Place logs on a lightly buttered or parchment lined cookie sheet.
Bake 25-30 minutes or until firm. Cool and cut diagonally into ⅜-inch slices. Place slices on a cookie sheet and bake at 250°F for approximately one hour to dry the biscotti.