Prepare brownie mix according to package directions, using a 13- x 9-inch baking pan. Bake at 350° for 20 minutes or until just set (brownies will be fudgy). Let cool completely.
Press raspberries through a fine sieve over a bowl, reserving 1 cup raspberry purée; discard seeds. Combine raspberry purée, sugar, cornstarch, and water in a saucepan; stir with a whisk until blended. Bring to a boil, and cook 1 minute or until thick, stirring constantly with a whisk. Pour raspberry sauce into a bowl; press plastic wrap onto surface of sauce, and let cool completely.
Add yogurt to raspberry sauce; swirl together, using a rubber spatula or spoon. Quickly spread yogurt mixture over brownies in pan. Cover and freeze 4 hours or until firm. Cut into 24 squares.