Newbie here. I made fudge frosting for my brownies and it's runny.
I believe it's a standard type of frosting recipe, made from sugar,
cocoa, milk, butter, and corn syrup. After cooling it said to mix
between 1/2 and 3/4 cup of powdered sugar in, along with some
vanilla. I mixed the maximum amount of powdered sugar in but it
was runny. I put it in the refrigerator to thicken it, but it
still ended up runny. What did I do wrong? I'm fairly strict with
the amounts, so I think I was close on that, and it said to boil
the first ingredients for 3 minutes and I set the timer and boiled
them for exactly that.

Short of buying frosting, are there any recipes for chocolate icing
that might be more foolproof than the recipe I used?

I also made Grasshopper brownies with a green mint layer that
doesn't look like it is going to set up. Expensive mistakes.

Thanks, Michael