Preheat oven to 375 degrees. Butter 3 6-oz. ramekins and dust with granulated sugar.
Cut the rind and pith from the oranges. Halve the oranges across the equator and take out any seeds, then cut into chunks and put them in a blender or food processor. Pulse until they're reduced to a smooth puree.
Pour the puree into a small saucepan, stir in the sugar and saffron and bring to a boil over medium heat, stirring occasionally to avoid sticking. In a cup or small bowl, mix the cornstarch and water to create a slurry. Stir the cornstarch mixture into the orange-sugar mixture over high heat. When it begins to boil, reduce the heat and simmer for 5 minutes. Transfer to a nonreactive bowl and let cool to room temperature.
Place the eggs whites and cream of tartar in a large bowl. Using an electric mixer, whip the egg whites at high speed until stiff peaks form. Gently fold in the orange mixture with a rubber spatula until blended (but don't stir too much to avoid losing the air in the egg whites).
Fill each ramekin to the top with the souffle mixture and smooth flat with a spatula. Place the ramekins on a baking sheet and set them on the top rack in the oven. Bake for 10 to 15 minutes until the souffles have risen about 1/2 inch above the ramekins and are golden brown on top. Remove from oven and dust with confectioners' sugar. Serve immediately.