Hi Pam. Macarons certainly are a wonderful addition to a wedding celebration! This scenario presents itself as more of a melting issue than a thickening issue. Instant ClearJel works by binding to water in a mixture, making it less likely to separate as those starches swell to thicken said mixture. It does not really bind to fat, so we are not sure that it would prevent any of the butter from melting by those higher temperatures you are expecting. If you do want to try incorporating some of the ClearJel into your frosting (which would be a bit of an experiment as we have not used this for Italian Buttercreams), we would recommend making a small test batch first (say a one cup yield). You can mix 1/2 teaspoon of the Instant ClearJel into one Tablespoon of powdered sugar. Then, mix that into a small amount of your finished buttercream. Once fully incorporated, you can then mix that into the remaining frosting. If you find that the test batch of stabilized buttercream works to your liking, you could increase the amount of ClearJel used in your full batch. For 5 cups of frosting, you could use 2 teaspoons of ClearJel mixed with 1/4 cup of powdered sugar. If you were to add the ClearJel during any other point in the process of making the frosting, it would likely thicken the syrup and/or the egg whites and alter the way the butter gets incorporated. 

We would recommend taking the macarons to the venue frozen and then allowing them to thaw on display. That can give you a bit of a head start for preventing melting and it would be our best suggestion to help give those delicate treats the attention they deserve. Please do let us know how it goes!

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