In a large bowl, combine the crushed graham crackers or digestive biscuits, cocoa powder, powdered sugar, and chopped nuts.
Add the sweetened condensed milk, melted butter, and vanilla extract. Mix until well combined.
Press the mixture into a lined 9x9 inch (23x23 cm) baking dish, smoothing the top with a spatula.
Refrigerate for at least 2 hours or until firm.
Cut into squares or bars and serve chilled.
Enjoy your meal!