Place the gelatin in a bowl of cold water, making sure that it is submerged and set aside for five minutes to soften. Meanwhile, melt the chopped chocolate either in short bursts in the microwave, or in a bowl over a pan of gently simmering water (make sure the base of the bowl doesn't touch the water.
Whisk together the milk and cocoa powder in small pan, heat until steaming and just starting to bubble around the edges, don't let it boil. Remove from the heat, squeeze the excess water out of the gelatin, add it to the milk and whisk until smooth.
Pour the hot milk into the chocolate and stir until smooth; allow to cool for a couple of minutes then mix in the yoghurt and vanilla extract.
Whisk the egg whites until soft peaks form, add the canderel sugarly (or sugar) then continue to whisk until the egg whites are stiff and glossy. Fold half of the egg whites into the chocolate mixture, then the other half. Divide the mousse between four to six glasses/jars/teacups and refrigerate overnight or for at least a couple of hours until set.