Place a steamer insert or mesh colander in a large pot and add enough water to reach the bottom of the steamer. Place the sweet potatoes in the steamer and bring the water to a boil. Cover the pot and lower the heat.
Cook over simmering water for about 25 minutes, until very tender. Check occasionally to be sure the water doesn't boil away.
Transfer the sweet potatoes to the bowl of an electric mixer fitted with the paddle attachment. With the mixer on low, slowly add the half and half, orange zest, orange juice, cayenne pepper, 1 T salt, 1 t black pepper.
With the mixer still on low, add the butter, 1 T at a time, until incorporated.
Taste for seasonings and mix until smooth. Serve hot.