Break milk and dark chocolate in a glass bowl and melt it in a water bath. Set aside.
In a separate bowl, cream butter, sugar and egg yolks. Beat with a hand held mixer.
Add the mixture into the chocolate and stir with a spatula until integrated.
Whip the cream with the hand mixer until thickened.
Add the chocolate into the whipped cream and mix very well.
Pour the milk into a deep dish and then dip the Maria cookies in it to soften them.
Line 8 inch cake pan with parchment paper.
Fill the cake pan with the mixture and biscuits, making sure you don´t leave gaps. The order is the following: biscuits - mixture - biscuits - mixture - biscuits - mixture.
Once you've assembled the cake, put it in the fridge overnight so that it's ready for the next day. Et voilà … Now you have a delicious chocolate biscuit cake.