In a large glass vessel, combine the coarsely ground coffee with cold, filtered water. Stir gently to ensure full saturation, then cover and allow to steep in the refrigerator for 12 hours, drawing out a smooth, rounded depth.
In a separate bowl, softly muddle the blackberries with the sage leaves and maple syrup, releasing their fragrant oils and rich colour. Divide this mixture evenly between two glasses.
Once steeped, strain the cold brew through a fine mesh sieve or coffee filter into a clean jug, yielding a clear, concentrated infusion. Discard the grounds.
Fill each glass with ice, then pour the cold brew over the blackberry and sage mixture, allowing the flavours to gently intertwine.
Warm the oat milk slightly, then whisk with maple syrup until smooth and lightly aerated, forming a soft, velvety foam.
Spoon the oat milk foam over each glass, letting it settle delicately on the surface. Garnish with fresh blackberries and a sprig of sage for a final, elegant flourish.
