If you are doing a partial boil, then it is recommended to do late additions of extract. However this is done more to reduce carmelization and improve hop utilization.
The way to reducing that twang in your beers is to cool the wort as quickly as possible, and maintain a steady fermentation temp. I brew both partial mash recipes and extract with grains recipes. Since I've paid more attention to rapid wort cooling and temp control, that twang has disappeared from my brews.