Combine coarsely ground coffee with cold water in a large jar. Stir thoroughly to soak all grounds. Cover and refrigerate for at least 12 hours to steep.

While steeping, rinse blackberries and sage. Muddle them with maple syrup in a bowl until juicy. Divide mixture into two glasses.

After steeping, strain the cold brew through a coffee filter or fine mesh strainer into a clean pitcher.

Fill glasses with ice cubes, then pour the strained cold brew over the muddled mixture, allowing the colors to swirl.

Warm barista oat milk in a small saucepan over low heat, whisking in remaining maple syrup until frothy. Do not boil.

Top each glass with frothed oat milk, garnish with fresh sage leaves and whole blackberries.