I tried making the frosting and it all seemed to be going well. I got the hard peaks (which took between 4-5 minutes). Then I added the butter....

I only just saw this *after* making the cake, agree with all the above. First time I have made frosting in this manner and patience is def needed - I think the whole process for me must have taken around 10 minutes of high speed whisking. There were times I thought it had gone wrong, but all of sudden it just "happened" and came together a treat!

This thread has been super helpful. I had a similar cooked buttercream (ermine frosting, specifically) experience a little while ago. That was the first time I had a recipe flop on me and the first time I've been discouraged to try making something again. Now I'm encouraged to try it again.

It's a tricky thing to time precisely. Unlike most buttercreams, this is a Swiss meringue. You should whip over the double boiler until the mixture is just warm to the touch and all the sugar granules are dissolved. Then, whip in the stand mixer to just stiff peaks. Before you add any butter, make sure the bowl is cool to the touch. Also, be sure your butter is not too warm. It should be just slightly soft. It is better to err on the side of too cold than too warm. It should take about 30 seconds for each piece of butter to be incorporated, but it will also depend on the temperature of your egg whites and your butter, as well as the speed of your mixer. Watch until there are no more chunks of butter before adding another piece.

The final texture should be like very thick sour cream or mayonnaise.

If it gets to thin and flat, you can just refrigerate to set the butter and then whip it until it has a nice thick texture and is spreadable.

Thank you for helping to clarify. So if my egg whites are too warm - does that mean I need to spend less time melting the sugar (ie less than the 3 minutes mentioned) or that I need to whip them longer, or take time to let them cool before adding the butter, or make sure the butter is colder? I've tried to research this on the internet, but the frosting recipe here is very different than most "cooked" frosting recipes.. I think the butter was fully incorporated, but that was hard to see, which is why I was wondering how long it should take for each piece (10 sec? 30 sec?).

It's possible that your egg whites were too warm and were melting the butter rather than emulsifying it into the mix. You should also make sure that each "chunk" of butter is fully incorporated into the frosting before adding another.