Preheat the oven to 350°F (175°C). Grease a 9-inch square baking pan and line the bottom with parchment.
In a medium bowl, whisk together all-purpose flour, ground ginger, ground cinnamon, ground cloves, and fine salt until evenly combined.
In a heatproof measuring cup, stir baking soda into boiling water until dissolved; it will foam.
In a large bowl, whisk unsulphured molasses, brown sugar, and egg until smooth; melt unsalted butter and whisk it in until glossy.
Whisk in the hot liquid from step 3 until evenly combined.
Add the wet mixture to the dry ingredients and whisk or fold just until a smooth, pourable batter forms; do not overmix.
Scrape the batter into the prepared pan and smooth the top. Bake until the top springs back to a light touch and a toothpick inserted in the center comes out clean or with a few moist crumbs, 30–35 minutes.
Cool in the pan on a rack for 15 minutes, then run a knife around edges and invert or leave in the pan to finish cooling.
Dust with powdered sugar (for serving), slice into squares, and serve warm or at room temperature.