Step 1: Prepare the Main Mixture

1. Soak the Oats: Combine the oat flakes and hot milk in a large mixing bowl. Stir well and let sit for about 5 minutes to soften the oats.
2. Mash the Bananas: Peel the bananas and mash them in a separate bowl until smooth.
3. Combine Ingredients: Add the mashed bananas, eggs, and vanillin (or vanilla extract) to the soaked oats. Stir until evenly combined.
4. Add Cocoa and Baking Powder: Sift in the bitter cocoa powder and baking powder, then mix until the batter is smooth and uniform.
5. Add Nuts and Chocolate Chips: Fold in the chopped walnuts and chocolate chips (if using) for extra texture and flavor.

Step 2: Bake the Bars

1. Prepare the Baking Pan: Preheat your oven to 180°C (350°F). Line an 8×8-inch baking dish with parchment paper, leaving extra overhang for easy removal.
2. Pour and Spread: Pour the batter into the prepared dish and spread it evenly with a spatula.
3. Bake: Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
4. Cool: Allow the bars to cool completely in the pan before adding the topping.

Step 3: Prepare the Chocolate Topping

1. Melt the Chocolate: In a heatproof bowl, combine the dark chocolate with the boiling milk. Stir until the chocolate is fully melted and smooth.
2. Spread the Topping: Pour the melted chocolate over the cooled bars, spreading it evenly with a spatula.
3. Set the Topping: Allow the chocolate to set at room temperature, or place the bars in the refrigerator for quicker setting.

Step 4: Slice and Serve

1. Once the chocolate topping has set, lift the bars out of the pan using the parchment paper overhang.
2. Transfer the bars to a cutting board and slice into squares or rectangles.
3. Serve as a healthy snack, dessert, or quick breakfast.