Heat oven to 350°F. Lightly butter an 8-inch square dish or six 8-ounce ramekins.
Toss cherries with granulated sugar, cornstarch, lemon juice, vanilla, and a pinch of salt. Fold in 1/4 cup chocolate chips and spread into the dish.
Combine oats, flour, brown sugar, cocoa, espresso powder, and remaining salt in a bowl.
Cut in the cold butter with a pastry cutter or fingers until moist clumps form. Stir in remaining chocolate chips and pecans, if using.
Cover the cherry layer with the crumble, leaving a few gaps for steam. Set the dish on a sheet pan to catch drips.
Bake until the filling bubbles at the edges and the top looks set and crisp, 32–38 minutes.
Cool 10–15 minutes to thicken the juices. Serve warm with vanilla ice cream or whipped cream.