Preheat oven to 325 degrees F (165 degrees C). Grease a 9x13-inch baking dish.

Mix flour, sugar, and butter together in a large bowl until large crumbs form. Separate 2 cups of the flour mixture into another bowl and set aside.

Stir boiling water, molasses, and baking soda together in a third bowl until baking soda dissolves; pour into the flour mixture in the large bowl and stir until smooth. Pour the batter into the prepared baking dish. Sprinkle the reserved flour mixture over the top of the batter.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 50 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack.