Preheat oven to 350 degrees. Butter a 13-by-9-inch baking pan. Line bottom with parchment, and butter parchment. Dust with flour, and tap out excess.
Stir together boiling water and baking soda in a small bowl, and set aside. Whisk flour, cocoa powder, spices, salt, and baking powder in a medium bowl.
Beat butter and brown sugar with a mixer on medium-high speed until fluffy. Add molasses and eggs, and beat until creamy, about 4 minutes, scraping down sides of bowl as needed. Reduce speed to low. Add flour mixture in 3 additions, alternating with the baking soda mixture, and beat until combined. Raise speed to medium-high, and beat for 15 seconds.
Pour batter into prepared pan. Bake until a toothpick inserted into center of cake comes out clean, about 40 minutes. (To prevent cake from falling too soon, do not open oven for the first 35 minutes of baking.) Let cool in pan on a wire rack for 20 minutes. Run a knife around edges of cake to loosen, and invert onto rack. Remove parchment, turn cake right side up, and let cool completely. (Cake can be wrapped in plastic and refrigerated for up to 3 days.)
Using a serrated knife, trim edges of cake so they are straight. Cut cake into 1-inch cubes. Cut 25 cubes on the diagonal into triangles. Spread frosting on long sides of triangles, and place one triangle on top of each cube, frosting side down, to create roofs. Insert a piece of licorice in top of each roof for the chimney. Dust roofs with confectioners' sugar. Petits fours are best the day you assemble them.