Chop dark chocolate finely, put it in a heatproof bowl and melt over a bain marie ( a pot of simmering water) stirring occasionally or in a microwave. (If using microwave make sure you heat it in 30 sec bursts, so it doesn't burn). Let it come to room temperature.
Measure and sift cocoa powder to get rid of lumps. In a medium-large bowl put very cold heavy cream and beat it using a stand or a hand mixer. You want something between soft and stiff peaks. (Not too soft, that it doesn't keep its shape and not to stiff like it's buttery almost). Add cooled melted chocolate and cocoa powder and mix just until everything is combined, you don't want to lose all that air you just beat into it.
Next, fold in condensed milk using a silicone spatula. Pour the ice cream mixture into a loaf pan or a any kind of pan you have, that is large enough. Mine was around 26x13cm (10x5 inch), but 9x5 inch will also do. For easier cleaning you can line the pan with parchment paper. Decorate the top with chocolate shavings.
Put the ice cream in the freezer for at least 4h hours. After that, dunk your ice cream scoop in some warm water and scoop away! Enjoy! :)