Preparation

• Position a rack in the center of the oven and heat to 350°F. Line a large rimmed baking sheet with parchment. Spread the streusel on the baking sheet and bake for 12 to 15 minutes, rotating the pan halfway through, until the streusel is golden brown and fragrant. Let cool to room temperature. (Use right away, or store in an airtight container for up to 5 days.)
• To assemble the sundaes, sprinkle about 1/4 cup streusel in the bottom of a sundae glass (or similar), then top with a scoop of ice cream, 1 Tbs. liquid from the jar of cherries, and two halved cherries. Repeat to make a second layer. Top the sundae with a dollop of whipped cream, a sprinkle of streusel, and a cherry.