Coat a 9-inch round cake pan with cooking spray; line bottom of pan with wax paper. Coat wax paper with cooking spray, and set pan aside.
Combine 3/4 cup brown sugar, vegetable oil, molasses, sour cream, dark rum and vanilla extract in a large bowl; stir well, and set molasses mixture aside.
Combine cornmeal, flour, baking powder, cinnamon, cardamom, ginger, nutmeg, cloves and salt in a bowl; stir well, and set aside.
Beat egg whites and cream of tartar at high speed of a mixer until foamy. Gradually add 2 tablespoons brown sugar, 1 tablespoon at a time, beating until stiff peaks form.
Gently fold egg white mixture and cornmeal mixture alternately into molasses mixture, beginning and ending with egg white mixture.
Pour batter into prepared pan. Bake at 350°F for 35 minutes or until a wooden pick inserted in center comes out clean; let cool for 10 minutes in pan on a wire rack.
Loosen cake from sides of pan, using a narrow metal spatula, and turn out onto a wire rack. Carefully peel off wax paper; let cool completely. Place cake on a serving plate; sift powdered sugar over cake. Serve.