1. Toss together cherries, sugar, and cornstarch in a medium bowl. Set aside; let cherries thaw slightly, about 35 minutes.

2. Preheat oven to 350 degrees. Using a pastry brush, generously brush one side of each slice of bread with the butter. Reserve 6 slices; cut remaining slices in half to form 36 rectangles. Line sides and bottom of each cup of a jumbo (6-cup) muffin tin with 6 rectangles, overlapping bread and placing buttered side against tin.

3. Toss cherry mixture to combine. Spoon into bread-lined cups. Top each with a reserved bread square, buttered side down; press firmly onto cherries.

4. Bake charlottes until tops and sides are golden and crisp, 35 to 40 minutes. Transfer tin to a wire rack; let stand until charlottes have cooled slightly and are just firm, 15 to 20 minutes. Using an offset spatula or knife, loosen sides of charlottes from tin, and unmold; invert onto serving plates. Serve warm, with warm sour cherry compote on the side.