Heat the liquid cream in the microwave or in a small saucepan. Pour it over the chocolate pieces. Cover and let melt for 3 minutes, mix well, then let cool for 4 hours.
After this time, whip the ganache. Mix milk and cocoa in a bowl.
Line a rectangular or square pan with cling film. Pour a first layer of ganache in the bottom of the mold. Smooth gently. Dip the cookies in the chocolate milk and place them in a second layer on top of the ganache. Cover with a new layer of ganache, then cookies, until you reach at least 2/3 of the mold.
Let stand 6 hours, or even overnight, in the refrigerator.
Unmold and sprinkle with a little powdered sugar if desired.
