He's wrong on so many levels it's amazing.
First of all, "schlag" is German for "slam. wham, smack" and I'm guessing it's slang for whipped.
Second, it's mocha, not cocoa which is the drink which comes mit schlag. Cocoa gets marshmallows, as everyone on the northeast coast knows.
And "mit schlag" is not a common phrase in NYC, which is where King is using it as if it's from there.
And it just does not match the tone of the other non-standard verbiage in the series.