Choose your container: Use a 12–16 oz jar or lidded container to allow room for stirring and toppings.
Mix the base: In the container, whisk milk, Greek yogurt, protein powder, cocoa powder, vanilla, salt, and sweetener until smooth. Break up any cocoa or protein clumps.
Add the oats: Stir in rolled oats (and chia or flax if using) until fully coated.
The mixture should look pourable, not stiff.
Swirl in hazelnut goodness: Dollop in the hazelnut butter or cocoa-hazelnut spread and gently swirl. You can fully mix it or leave streaks for pockets of flavor.
Chill: Cover and refrigerate for at least 4 hours, ideally overnight, until thick and creamy.
Adjust texture: In the morning, stir and add a splash of milk if you prefer a looser consistency.
Finish with toppings: Add chopped hazelnuts, a few dark chocolate shavings, and fruit. Enjoy cold, or warm briefly in the microwave if you like.