Preheat oven to 350°.
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.
Put 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.
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.
Serve with whipped topping, if desired.