If anyone every tries to tell you how to make "buttercream" without icing sugar run in the other direction. They are lying. Its gross.
I decided I wanted to make a cake but totally overlooked the fact that I only had a cup of icing sugar in the house. So I googled to see if there was some way to make an icing similar to buttercream without it. I found a recipe that was 4.5 tablespoons of flour, 1 cup of milk and 1 cup of granulated sugar. Cooked over low heat until thick, beat in stand mixer until room temp and then beat in a cup of butter.
As you can see the texture was...bleck. It looked like a glaze after I had made it but once it was on the cakes it looked curdled/seperated/like it escaped from hell. Tasted ok...for what it was.
I couldn't bring myself to even try a piece. I sent Will to the store the next day for more butter and icing sugar and made a proper buttercream
So good. Never fuck with a classic. Just go buy more icing sugar. It's worth it.