Homebakers -im having donut issues

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I bake my donuts. Let them cool at least 1 hr. I powder them with 2 tbsp cornstarch and 1 c powered sugar. The sugar always dissolve and becomes gooey! I tried leaving them in a cooled oven on a cooling rack over night, I have packaged them in clear cookie bags, boxes, you name it I've tried it. How can I make a simple powdered donut that will stand up to packaging to sell at farmers markets?