Make them bigger

Of course, a flat chest simply won't do for what Duromast has in mind, so the first order of business is to increase their size. He finally releases her nipples, letting her stand there still with her arms held behind her head. He takes a large red bottle from his desk and pours some of the think creamy liquid within into his hand. Then, putting one hand on Cinnamon's back to hold her still, he slaps the cream filled hand hard against her chest. The slap stings badly, hard enough that she'd have been knocked over if he wasn't holding her up. Cinnamon yelps and squirms as he vigorously begins rubbing the cream into her breasts, sliding between one and the other. The cream immediately begins to burn and itch, and her breasts throb as they start to slowly grow.

Again he takes the red flask off of the desk and dribbles another layer of the thick liquid across the tops of her now b-cup sized breasts, and again he begins to forcefully rub the cream into the flesh of her breasts. His huge hands smashing her breasts flat with each pass. Once the cream is fully rubbed in he begins spanking and slapping her growing tits. The potion, when brewed, was designed to target only the breasts. Still, her nipples too are now noticeably larger as well, each hard nub extending a little over an inch long.

Duromast finally stops working the poor girl's tits over and takes a moment to inspect them. They've easily grown to a healthy D cup, firm and round with obscenely thick and long black nipples. Her breasts are a dark red color from the effects of the potion as well as the spanking, but should revert back to their rich brown color soon.