In a 10- to 12-inch nonstick frying pan, frequently shake sugar over medium heat until all is melted and the color is deep amber, 3 to 5 minutes. Immediately pour into an 8- to 9-inch diameter straight-sided baking dish or pan (at least 1-qt. size). Quickly rotate dish to coat bottom. In a 1- to 1 1/2-quart pan over medium-high heat, stir milk and caramel powder until all the caramel melts (it hardens first), about 7 minutes. In a bowl, whisk some hot liquid with eggs and vanilla; stir back into pan. Remove from heat. Pour mixture into prepared baking dish. Set dish in a larger rimmed pan, pour 1 inch boiling water into outer pan. Bake in a 350F oven until center of flan jiggles only slightly when gently shaken, 35 to 45 minutes. Remove dish with flan from water. Let cool, then cover and chill for at least 3 hours or up to 2 days. To serve, run a thin knife between flan and dish. Invert platter over flan; holding together, invert. Lift off pan. Cut into wedges.