Beginner Gardening: Yellow Flowering Ground Cover, 1 by FlowerGirlMO

FlowerGirlMO wrote:
I have this planted in one of my gardens in St. Louis, MO. I got it from a friend of friend who lives in Arkansas. My friend said that is was some kind of ground cover, but neither of us can find the name of it. It is about 12 inches high and grows on an individual stem with a single flower at the top. But it has more than doubled in size/number of plants since I planted it last spring.

Please help.