Grilling the cake slices is optional but makes for a lovely presentation. You can bake the cake a day ahead and grill just before serving. This cake is prone to sinking slightly in the center, but once it's sliced, no one can tell. Serve with whipped topping, if desired.
To prepare cake, coat an 8 x 4-inch loaf pan with cooking spray; set prepared loaf pan aside.
Lightly spoon all-purpose flour into dry measuring cups; level with a knife. Combine flour, polenta, baking powder, and salt in a medium bowl, stirring with a whisk; set aside.
???,,vanilla extract and whole eggs in a small bowl, stirring well with a whisk. Combine 1 cup sugar and butter in a medium bowl; beat with mixer at medium speed until mixture is light and fluffy (about 5 minutes). Gradually add vanilla extract mixture and egg whites to sugar mixture, beating well (about 2 minutes). Gradually add flour mixture, and stir batter just until moist.
Pour cake batter into prepared pan. Bake at 350° for 1 hour or until a wooden pick inserted in center comes out clean. Cool cake in pan 10 minutes on a wire rack; remove cake from pan. Cool completely on wire rack. Cut cake into 9 equal slices.
To prepare berry topping, combine 1 tablespoon sugar, water, and thawed raspberries in a food processor or blender; process until the mixture is smooth. Strain mixture through a sieve over a bowl, pressing with the back of a spoon; discard solids. Combine raspberry sauce, strawberries, blueberries, fresh raspberries, and blackberries in a large bowl; toss gently to combine.
Prepare grill to medium-high heat.
Place cake slices on grill rack coated with cooking spray; grill 1 minute on each side or until golden brown and grill marks form. Serve grilled cake slices with berry topping.