I've made Sugarshack's buttercream several times. I think I remember it crusting the first time or two but lately it hasn't. I know it's summer but our A/C is always set on 74 so humidity shouldn't be an issue, right? I have iced the last few cakes cold (not frozen but from the fridge) because the icing seems to go on easier without pulling off the crumb coat. Maybe if I understood what made a buttercream crust, I could better understand why mine isn't?
The higher the sugar to fat ratio the more crusting a butter cream will be.
Have you been adding more liquid than usual? Is it thinner than you've made it in the past?
I don't use her bc but one of my bc's is a crusting ps base all butter recipe and it can be temperamental in the summer, even in A/C.
After you ice are you leaving it out at room temp or putting it back in the fridge?
I recommend leaving it out at room temp.
I'm not a chemist, but I would guess it's the sugar which makes the bc crust. Lately I've been having issues with the same thing but I really think it's the summer heat and humidity. (98* outside, 88% humidity, plus an air conditioner that doesn't work well.)
I have tried a half a dozen different 'tweaks', this summer to get it to crust.
For me, cutting back on the liquid and adding some meringue powder has helped. I have had to cut my liquid back as much as half. If I add too much meringue powder, it makes my bc too dry and crumbly.
I have tweaked my Wilton Recipe to be similar to sugar shacks. I use the creamer instead of milk and I have found that almost over loading the KA mixer helps cut back on air bubbles.
I am looking forward to fall and cooler dryer weather.
I have also found that if my cakes are too cold, it causes condensation and turns my bc into a mess, and it seems as if I can never get the dern thing iced, dried or smoothed!
Now, I torte them in layers, seperate them, and let them sit until they are almost room temperature. I use an icer bag so I don't knock them to bits with my heavy hand.
I hope this helps!