Semi-randomly rummaging through Google Books due to another freeform Making Light thread.
The "Knickerbocker Glory" layered ice cream concoction seems to've originated in the US and was carried over to the UK by the 1920s, perhaps via American doughboys during/after the Great War. Oddly, it lived on in the UK despite being eventually forgotten in the US (at least by that name), so that the term was considered sufficiently exotic to be "translated" in the US editions of "Harry Potter".
The earliest ref I've found so far is from the January 1909 issue of the Ice Cream Trade Journal, published in the US: the "Knickerbocker Sundae", composed of vanilla ice cream, chocolate syrup, crushed raspberries, whipped cream, marachino cherries, and rose syrup.
This may or may not be related to the Knickerbocker "fountain chocolate" company, which was providing sundae recipes by 1908 (although none of the ones in that ref have the raspberry/rose ingredients.
It does predate the 1915 ref cited here, which has the same general list of ingredients and emphasizes the layered construction.
On edit: Hah-- found an even earlier one from The Practical Druggist, June 1903: same ingredients *and* established emphasis on the layers.
This 1914 ref explicitly describes "the Knickerbocker" as "a Sundae from New York". There's also some (not nec'ly exclusive) possibility that the "Knickerbocker" name is partially based on the use of raspberries, as there seems to be an established mixed drink called the "Knickerbocker" that uses raspberry syrup; I've found a fleeting and mostly unsnippeted 19th(?) century reference to "the great Central Hudson raspberry belt" which "nightly shipped to New York its 3,000 baskets [of raspberries] to tickle Father Knickerbocker's palate next morning"-- perhaps there's an old connection between NYC and raspberries?