Add the oat flour, chocolate hazelnut protein powder, cocoa powder, yogurt, and water or milk to a bowl.
Mix until smooth and well combined.
Add the chia seeds last and mix thoroughly to prevent them from clumping together.
Cover and refrigerate for 2–4 hours, or leave overnight for a thicker texture.
Once set, spread the hazelnut butter over the top.
Melt the dark chocolate in the microwave in short bursts, stirring between each one.
Drizzle the melted chocolate over the oats to create a thin chocolate shell.
Sprinkle with chopped hazelnuts.
Refrigerate again until the chocolate has completely set, then enjoy!