Eef Tulp opened Dubbel Dutch, a Dutch grocery and sandwich shop in Denver, 19 years ago. At the time, her store was one of the only places in the region to buy stroopwafels.

Vinyl banners in the windows of a narrow retail space at Petaluma’s Deer Creek Village shopping center showed images of the delectable European treat named with the Dutch words for “Syrup waffle,” a ...