That .EXE is automatically signed with certificate that you are signing your main setup.
Indeed, without signing the package, you will be prompted with the UAC as shown in your screenshot. However, if your package is signed with a certificate, the TempIISBrowseProxy.exe will also be signed, as shown in the below screenshot:
If you want to avoid multiple UAC prompts, you should be running the setup elevated from the start. The browsing also requires elevated rights, this being the reason why there are multiple UAC prompts.
Hope this helps.