Step 1: Prepare the Cake Base

1. Crush the Oreos: Start by crushing the 10 Oreo biscuits. You can do this by placing them in a zip-lock bag and using a rolling pin to crush them finely. Alternatively, you can use a food processor to make the process quicker and easier.
2. Mix with Butter: In a small bowl, mix the crushed Oreos with the melted butter until well combined.
3. Form the Base: Press the Oreo mixture evenly into the bottom of a 4-inch cake ring to create the base of your cake. Once pressed, chill the base in the refrigerator while you prepare the chocolate mousse layer.

Step 2: Make the Chocolate Mousse

1. Melt the Chocolate: Break 200 g of semi-sweet dark chocolate into small pieces and place them in a heatproof bowl.
2. Heat the Cream: In a small saucepan, heat 1/3 cup (80 ml) of whipping cream over medium heat until it just begins to simmer. Remove from heat immediately.
3. Combine Chocolate and Cream: Pour the hot cream over the chocolate pieces and let it sit for a minute to soften. Stir gently until the chocolate is completely melted and smooth. Allow the mixture to cool slightly.
4. Whip the Cream: In a separate bowl, whip the remaining 300 ml of whipping cream until stiff peaks form.
5. Fold in the Whipped Cream: Once the chocolate mixture has cooled slightly, gently fold the whipped cream into the melted chocolate mixture in small batches. Be careful not to deflate the whipped cream, as this will give the mousse its light and airy texture. Continue folding until the mousse is smooth and fully combined.

Step 3: Prepare the Chocolate Mixture

1. Melt the Chocolate: Break 100 g of semi-sweet dark chocolate into small pieces and place them in a heatproof bowl.
2. Heat the Cream: Heat another 1/3 cup (80 ml) of whipping cream until it is hot (just before boiling), then pour it over the chocolate pieces.
3. Stir Until Smooth: Let the cream and chocolate sit for a minute, then stir gently until the chocolate is completely melted and smooth. Set aside to cool slightly. This will form the top chocolate layer of the cake.

Step 4: Assemble the Cake

1. Layer the Mousse: Remove the Oreo base from the refrigerator. Pour the prepared chocolate mousse over the chilled Oreo base, spreading it evenly with a spatula to ensure a smooth layer.
2. Add the Chocolate Mixture: Carefully pour the cooled chocolate mixture over the mousse layer, spreading it evenly with a spatula for a glossy finish.

Step 5: Chill and Serve

1. Chill the Cake: Place the assembled cake in the refrigerator and allow it to chill for at least 4 hours or until fully set. This step is crucial for allowing the mousse to firm up and the flavors to meld together.
2. Remove the Cake Ring: Once the cake has chilled and set, carefully remove the cake ring. You can gently run a knife around the edge of the cake to help release it from the ring.
3. Decorate: If desired, decorate the top of the cake with sprinkles or other toppings of your choice for a festive touch.
4. Serve: Slice the cake into portions and serve chilled. Enjoy your rich and creamy Oreo Chocolate Mousse Cake!