Shuck corn and remove kernels. Reserve ears in an 8-quart stockpot and cover with water. Bring to a boil and let simmer for 1 hour. Strain. Reserve liquid and discard cobs. Place a large saucepot over medium-high heat; add butter and onions, and then sauté mixture for 3 minutes, or until translucent. Add corn and corn broth; bring to a boil. Let simmer for 15 minutes. Pour in whipping cream and bring to a boil. Reduce heat and let simmer for 45 minutes, or until thickened. Just before serving, whisk in cold sweet butter and season with salt and pepper.