1. Preheat oven to 350 degrees. Cut two 9-inch squares of parchment paper to line the bottoms of two 9-inch square baking pans. Butter pans; line with squares of parchment. Butter linings and then flour pans, tapping out excess. Set pans aside.

2. Stir together baking soda and boiling water; set aside. Whisk flour, spices, salt, and baking powder in a medium bowl.

3. Put butter and brown sugar in the bowl of an electric mixer fitted with the paddle attachment. Cream on medium speed until fluffy, about 4 minutes. Mix in the molasses and eggs, scraping down sides of bowl, until creamy, about 4 minutes. Reduce speed to low. Add flour mixture in three batches, alternating with two batches of the baking-soda mixture; mix until flour is incorporated. Raise speed to high; mix until all ingredients are well combined, about 15 seconds. Stir batter several times with a rubber spatula, scraping the bottom of bowl.

4. Divide batter between prepared pans. Bake until a cake tester inserted into centers comes out clean, about 40 minutes (do not open oven door during baking). Let cool in pans on wire racks for 20 minutes. Run a thin knife around sides of cakes to loosen; unmold. Peel off parchment. Let cool completely on racks.