Place first 3 ingredients in a 9-inch round cake pan coated with cooking spray. Place pan over medium-low heat until sugar and butter melt, stirring frequently. Arrange tomato slices in a single layer over sugar mixture. Set aside.
Preheat oven to 350°.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, and next 5 ingredients (baking powder through salt); stir with a whisk. Place granulated sugar and 5 tablespoons butter in a large bowl; beat with a mixer at medium speed until blended. Add molasses and egg yolks, 1 at a time; beat well after each addition. Beat in vanilla. Add flour mixture and milk alternately to sugar mixture, beginning and ending with flour mixture.
Beat egg whites with a mixer at high speed until stiff peaks form. Fold egg whites into batter; pour batter into prepared pan. Bake at 350° for 45 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 15 minutes. Place a plate upside down on top of cake, and invert onto plate. Cool completely. Sprinkle with powdered sugar.