Lightly grease a 9 inch springform pan with cooking spray and set aside.
In a food processor crush all the biscuits while leaving also a good amount of biscuits bigger.
In a large bowl, add the crushed biscuits, chopped nuts, cocoa powder, melted butter, sugar and milk. Combine together until you get a sticky mixture.
Place the mixture into the pan, and press it down to create an even layer. Store in the fridge while making the ganache.
Chop finely the chocolate and place in a bowl. The finer the chocolate is, the quicker it will melt. Gently heat the heavy cream on medium heat until it starts to simmer. Pour the cream over the chocolate and let is rest for about 3 minutes. Once most of the chocolate has melted, stir until completely combined. Let it rest at room temperature for about 5 minutes.
Pour the chocolate ganache over the chocolate biscuit base in the pan. Spread it evenly to create a smooth layer.
Store in the fridge for about 6 hours or overnight, so that the cake sets well.