1. Line an 8-inch or 9-inch square baking pan with parchment paper, leaving some overhang for easy removal.

2. Place the digestive biscuits in a food processor and pulse until finely crushed. Alternatively, crush them in a sealed bag with a rolling pin.

3. In a large mixing bowl, combine the crushed biscuits, crisp rice cereal, and cocoa powder.

4. Place the butter, granulated sugar, milk, cocoa powder, and salt in a medium saucepan. Heat over low to medium heat, stirring frequently, until the butter has melted and the mixture is smooth.

5. Remove the saucepan from the heat and stir in the vanilla extract.

6. Pour the warm chocolate mixture over the crushed biscuits and crisp rice cereal. Gently fold until everything is evenly coated, taking care not to crush the cereal.

7. Transfer the mixture to the prepared baking pan and press it firmly and evenly into the pan using a spatula or the bottom of a glass.

8. Place the base in the refrigerator while preparing the chocolate icing.

9. Place the chopped chocolate in a heatproof bowl. Heat the heavy cream in a small saucepan until hot and steaming but not boiling.

10. Pour the hot cream over the chocolate and let it stand for about 2 minutes.

11. Add the butter and whisk gently until the chocolate is completely melted and the icing is smooth and glossy. Stir in the vanilla extract.

12. Pour the chocolate icing over the biscuit base and spread it evenly with a spatula.

13. Refrigerate the cake for at least 2–3 hours, or until the base is firm and the chocolate icing has set.

14. Lift the cake from the pan using the parchment paper. Cut into squares or bars with a sharp knife and serve.