Preheat the oven to 350°F. Arrange 8 (6-ounce) ramekins in a 9-inch by 13-inch baking pan and place in the oven to heat.
Cook 1 cup of the sugar, 1/4 cup water, and the cream of tartar in a large frying pan over medium-high heat, stirring occasionally, until the sugar turns deep golden and a caramel forms, 8 to 12 minutes. Remove the ramekins from the oven and pour about 1 tablespoon of the caramel inside each dish. Tilt the dishes until the caramel thickens and evenly coats the bottoms and 1 to 2 inches of the sides.
Bring a kettle of water to the boil. In a bowl, whisk the remaining 2/3 cup of sugar with the remaining ingredients (except the whipped cream and graham crackers).Pour the mixture into the ramekins.Set the baking pan onto an oven rack. Pour enough boiling water into the pan to come halfway up the sides of the ramekins.
Bake until the center of the flans barely jiggles and a knife inserted in the center comes out clean, about 25 minutes.Remove the ramekins from the water and cool on a rack, then chill, covered, at least 4 hours.
Run a thin metal spatula around the inside of each ramekin, then invert onto a deep plate. Set the the ramekins in hot water for a few minutes to loosen the remaining caramel (not all of it will come off); scrape the caramel over the flans. Serve with a little fresh whipped cream and crumbled graham crackers.