Cake: Stir together flour, sugar, baking powder and salt. Add milk, butter and vanilla; beat at low speed just to blend. Beat at medium speed for 2 minutes, scraping down side and bottom of bowl twice. Add eggs; beat for 2 minutes at medium speed. Grease and flour 9-inch (2.5 L) square cake pan; line with waxed paper. Spread batter evenly in pan; bake in 350 degree F oven for 45 to 55 minutes or until tester inserted comes out clean. Let cool in pan on rack for 10 minutes; turn out of pan to cool completely on rack. (Cake can be stored in airtight container for up to 1 day or frozen for up to 1 month).
Slice berries and combine with sugar; let stand at room temperature for 1 hour. Whip cream. Slice cake into 2 layers; place bottom layer, cut side up, on serving plate. Reserve 2 cups berry mixture for topping. Spoon remaining berry mixture over bottom layer; spread with about one third of the whipped cream. Top with second layer, cut side down. Ice sides and top with remaining cream. (Cake can be stored in refrigerator for up to 1 day.) Spoon reserved berries over each serving.