Fill a medium saucepot with water and place over high heat. Bring to a boil and season with salt. Drop the carrots into the boiling salted water and cook until tender. Drain them and place into a blender or food processor. Add the stock and purée.
While the carrots are cooking, fill a large pasta pot with water and place over high heat. Once the water comes to a boil, add some salt and drop the penne. Cook to al dente according to package instructions.