In the bowl of your blender, put half of the milk, cocoa powder and sugar. Blend until the mixture is smooth.
Put the preparation in a saucepan and heat it until it simmers. While stirring, add the remaining milk. Stir until the sugar is completely dissolved and then let it cool.
Once the preparation is cold, put it in the blender, adding the yoghurt and vanilla extract. Blend with the "smoothies" function or at low speed until the mixture is smooth and homogeneous. Put in the freezer until the edges solidify.
Take the preparation out and mix it again in the blender at medium speed. Once smooth, freeze again. Repeat 5 more times and let it solidify. It's ready now !