In a jar or bowl, whisk the coffee, milk, vanilla, and sweetener until smooth.
Add the protein powder and whisk until no lumps remain.
If it clumps, let it sit 1–2 minutes, then whisk again.
Stir in the Greek yogurt, oats, chia seeds (if using), and a pinch of salt.
For a tiramisu touch, fold in the mascarpone gently. This adds richness without overpowering the coffee.
Cover and refrigerate at least 4 hours, ideally overnight, until thick and creamy.
Before serving, give it a stir. Adjust with a splash of milk if too thick, or a touch more sweetener if needed.
Top with a light dusting of cocoa powder and a sprinkle of shaved dark chocolate or cacao nibs.