Place the whole marionberries in a medium saucepan with the granulated sugar. Place over medium heat until the juices have rendered from the berries (about 10 minutes). Stir vigorously making sure as much juice comes out as possible. Pour over a sieve separating the juice from the seeds.

Place the berry juice with the basil in a blender (or just use an immersion blender if you have one) and blend until the basil is almost unnoticeable.

Let the mixture come to room temperature before adding the powdered sugar one cup at a time until it becomes a glaze consistency.