To make the knit heart when you don't plan to felt it, work it in garter stitch instead of stockinette.
A heart replaces the tassel on this little girl's hat. It was knit by the same method as the top down knit heart, using all knit stitches instead of purling on the back, giving it a sturdier shape. It was knit on smaller needles, using DK weight yarn. The pattern was modified slightly to make a smaller motif --- and if I remembered what I did I would tell you how I did it.