That would be "en glass". You can count ice cream in Swedish if you're talking about ice cream cones or things like this,whatever you call them:
The thing above could be called just en glass in Swedish, or to specify, en pinnglass. An ice cream cone is en glasstrut (cone = strut).
the glass = glaset. This is an ett word: ett glas, glaset, plural glas, glasen.
But the ice cream word is an en word: en glass, glassen, plural glassar, glassarna
You can also use the ice cream word as a mass noun of course.
The definite singular ending is always either -en or -et, there will never be two t in there.