Preheat oven to 350 degrees. Lightly coat an 8-inch square baking dish with cooking spray. Ina small bowl, whisk together flour, baking soda, & salt. In a large bowl, using an electric mixer, beat butter & granulated sugar on med-high until pale & fluffy. 4 minutes. With mixer on medium, add molasses & egg whites & beat until combined. With mixer on low, add flour mixture & beat until combined.
Transfer batter to dish & smooth top. Bake until a toothpick inserted in center of cake comes out clean. 40-45 minutes. Let cool completely in dish on a wire rack. Invert cake onto a cutting board & cut into 1 1/2-inch squares. (Store uncoated squares in an air-tight container at room temperature, up to 3 days). To serve, toss in confectioner's sugar to coat.