In a bowl, cream butter and sugar until fluffy. Beat in egg and vanilla. Combine the flour, baking powder and salt; add to creamed mixture alternately with milk.
Pour into a greased and floured 9-in. round baking pan. Bake at 350° for 20-25 minutes or until a toothpick inserted in the center comes out clean. Cool for 5 minutes before removing from the pan to a wire rack to cool completely.
Meanwhile, for filling, heat 1 cup raspberries in microwave for 30 seconds, then mash and measure 1/4 cup juice. Discard seeds.
In a small saucepan, combine sugar and cornstarch; gradually add raspberry juice. Bring to a boil; cook and stir for 1 minute or until thickened. Remove from the heat. Stir in lemon juice and vanilla; cool.
In a bowl, combine sour cream and whipped topping. Fold in cooled raspberry mixture and remaining berries. Cut cake widthwise in half. Place one layer on a serving plate. Spread with half of the filling. Top with remaining layer and filling. Garnish with additional berries if desired. Store in the refrigerator.