Sift the flour, cocoa, baking soda and salt, if using. Set aside.
Melt the chocolate and butter together in a large heat-proof mixing bowl. You can do this either over a pot of simmering water on the stove, or in the microwave in 10-second intervals, stirring between each. Set aside to cool.
Preheat the oven to 220 C and line 12 muffin moulds with liners or grease them. Liners are recommended because these muffins are quite fragile.
Now stir the sugar into the cooled chocolate mixture, followed by the eggs and vanilla. You can beat them in by hand or use a hand mixer.
Once the eggs are properly mixed in, stir in the yoghurt. Finally, fold in the flour mixture. If the batter seems a little uneven, run a hand mixer through it to smoothen it out.
Fold in the chocolate chips, then divide the batter equally among the muffin moulds and bake for 5 minutes at 220 C to help them get an initial rise, then reduce the temperature to 175 C and continue baking for about 15 minutes till the muffins have developed a crust and a toothpick poked in the center comes out clean.
Allow the muffins to cool completely on a wire rack, then dig in! Store them at room temperature in an airtight tin for 3 to 4 days, although in humid environments, I recommend transferring them straight to the fridge. Happy baking!