Prep your dish: Lightly grease a small oven-safe ramekin or mini baking dish (8–10 oz). Preheat the oven to 350°F (175°C).
If microwaving, skip preheat.
Blend or mix oats: For a cake-like texture, blitz oats in a blender for 10–15 seconds to make oat flour. For more chew, leave them whole.
Combine dry ingredients: In a bowl, stir oats, protein powder, baking powder, cocoa (if using), and salt until evenly mixed.
Add wet ingredients: Whisk in milk, sweetener, vanilla, and Greek yogurt/applesauce. The batter should be pourable but not watery.
Adjust with a splash of milk if too thick.
Fold in the Mars Bar: Stir in most of the chopped Mars Bar, reserving a few pieces for the top.
Transfer and top: Pour batter into the dish and scatter remaining Mars Bar pieces on the surface.
Bake or microwave: Oven: Bake 16–20 minutes, until the edges are set and the center is just slightly soft.
Microwave: Cook 1 minute 30 seconds to 2 minutes, checking every 20–30 seconds. Stop when the top is set but still moist.
Rest and serve: Let it sit for 2–3 minutes to settle. Add optional toppings and enjoy warm.