Hi there, Kavita! We don't think adding cornstarch will help much with this situation since there is already cornstarch in the confectioners' sugar. American buttercream isn't the ideal choice because it's going to absorb ambient moisture from the air. We'd recommend trying it again with an Italian buttercream using 8 ounces of shortening for 8 ounces of the butter in the recipe. Instant ClearJel is a cold-process thickener, so it doesn't require heat. You can learn more about this product here. The glaze you mentioned that is used on fresh fruit tarts is a bit different, often it's honey, jam, or simple syrup that hass been thinned a bit before brushing the fruit. We hope this can help! 

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