I've made a decent amount of swiss meringue buttercreams before, I'm not a pro but feel like I understand the basics at least. I've tried twice to make it with brown sugar, the second time I did half granulated and half brown. I whip it to stiff peaks and a nice cold bowl, then add softened butter. This time, I added less than a tablespoon and the whole thing melted....again! The taste is amazing, kind of reminds me of a pretzel. I really want to figure out how to get this right

I put it in the fridge to get really cold...nope, still soupy I put it in the freezer.....soupy I beat until I thought my mixer would burst into flames...soupy I added powdered sugar....this was a terrible idea, irreversibly ruined the whole thing!

Has anyone had success with this? Any suggestions?