Butterbeer, chocolate frogs or cauldron cakes, and the Simpson's big pink donut are probably the most obvious ones. The cauldron cake is really cute and can be found in both Diagon Alley and Hogsmeade iirc. The chocolate frogs come with collectable wizard cards just like in the books/movies. There's also never-melting ice cream which is also super cute (but far too sweet for me as a grownup as it's basically frosting in a cup haha).
But depending on your favorite characters/IP, there are several themed desserts out there flying under the radar including Minions and Seuss desserts in US and IoA respectively. There are several of these treats in the parks but they're rarely mentioned; they don't get the same kind of coverage that Disney treats do, for whatever reason.
And, not themed per se, but I do love a Universal churro and the Today Cafe has cheesecake on a stick which is fun.
Depending on what's in the Tribute Store when you go, you'll likely find something fun there too!