Combine 1 cup of milk, vanilla extract and sugar in a saucepan. Whisk until smooth, heat until boiling over medium-high heat. Mix the rest of the milk with cornstarch and raspberry powder in a small bowl, until well combined and smooth. Pour the raspberry mixture into the boiling milk in a saucepan. Cook over medium heat, stirring constantly until pudding is very thick and smooth (about 5 minutes). Pour into serving dishes and serve warm or chilled. This pudding can be used as a warm sauce over chocolate or vanilla cake.