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Managed to find it based on your description and Google Streetview.
When I was in Rome I went to Frigidarium (which is about 2 blocks further west than the one you went to), Claudio Torcé’s Il Gelato (near Circo Massimo) and Gelateria Fatamorgana (in Monti, close to the hotel I was staying and also close to the Cavour subway stop). All 3 were excellent, but I found Fatamorgana the best because they also made somewhat less conventional ice creams. This was over the course of 2,5 days.