Thaw and drain the frozen red raspberries well, reserving 1/3 cup of syrup. Pick over the fresh berries. In a heavy saucepan, combine the sugar and cornstarch, add the reserved syrup and lemon juice, and combine the mixture well. Add the thawed fruit and the fresh black raspberries, bring the liquid to a boil over moderate heat, stirring constantly, and simmer the mixture for 5 minutes. In a large bowl, cream the butter. Add the sugar, a little at a time, beating and beat the mixture until it is light and fluffy. Add the egg and vanilla, and beat the mixture until it is smooth. Into a bowl, sift together the flour, baking powder, and salt. Add flour mixture to the butter mixture in small batches, alternating with additions of milk. Beat well after each addition and blend the batter until it is smooth. Spread the batter evenly in a lightly buttered 9-inch square baking pan. Spoon the berry mixture over it. Bake in the middle of a preheated 350f oven for 20 minutes. Reduce heat to 325f and bake for 20-25 minutes more or until tester comes out clean. Let it cool on a rack for 10 minutes. Serve with cream or whipped cream. a 1983 Gourmet Mag. favorite by Nancy Bagget