Butter and line a 9 x 9-inch pan with parchment paper.
Preheat oven to 350 F.
Cream together butter and sugar. Separately combine baking soda and molasses.
In another bowl, sift flour with baking powder, spices, and salt. Stir molasses into butter mixture. Add flour mixture and boiling water alternately, mixing well after each addition. Beat in egg and stir in crystallized ginger.
Scrape batter into pan and spread gently to level. Place cranberries on top of cake batter and press in slightly. Bake for 35-40 minutes, until a tester inserted in the center of the cake comes out clean. Allow to cool for 20 minutes.