Peel the sweet potato and chop it into pieces about 1/2" thick and 1" wide and set it aside.
Trim the roots and cut the greens off of the leeks about 1 inch from where it starts turning from white to green. Slice the leeks in half then chop them about 3/8 of an inch wide.
in a large frying pan (preferably cast iron) on medium high heat, melt the butter and add the leeks and garlic. Stirring constantly for about 5 minutes, sauté the garlic and leeks until the leeks turn clear.
Slowly add the chicken stock, the sweet potato chunks, the white pepper, and the salt.
Bring the mixture to a boil and reduce the heat to medium. Cover the pan and let simmer until the potato chunks are soft.
once the potatoes are soft, transfer the mixture to a blender and blend until the mixture is consistent in texture (approximately 2 minutes)
Add the heavy cream and the brown sugar and continue to blend.