Marshmallowy buttercream tops the crunchy crust and tender interior of this rich cake. Fresh blackberries add a lush fruitiness and tame the sweetness with their slightly tart flavor.
Cooks' notes:
•Jam keeps, chilled in an airtight container, 2 weeks. Bring to room temperature before using.
•Cake layers can be baked 2 days ahead and kept, wrapped in plastic, at room temperature.
•Buttercream can be made ahead and chilled, covered, 1 week or frozen 1 month. Bring to room temperature (do not use a microwave) and beat with electric mixer until smooth before using.
•Assembled cake keeps, loosely covered, at room temperature 1 day.