Combine coarsely ground coffee and cold water in a large jar. Stir gently, cover tightly, and refrigerate for at least 12 hours for maximum flavor extraction.

In a bowl, muddle fresh blackberries with sage leaves and maple syrup until the berries are well broken down. Divide this mixture evenly between two serving glasses.

Use a coffee filter or fine mesh strainer to carefully remove the coffee grounds from the cold brew concentrate.

Fill each glass with ice cubes until just full, then pour the freshly strained cold brew over the muddled berry mixture.

Warm the oat milk gently and whisk in maple syrup until it becomes smooth and frothy.

Spoon the frothed oat milk generously over each glass of cold brew and garnish with additional sage leaves and whole blackberries.