Preheat the oven to 175 C. Grease a 9" round cake tin (I prefer springform for easy removal).
In a small bowl, stir the cocoa and hot water till combined. Set aside.
Now, in a mixing bowl, beat the butter and sugar with a hand mixer on medium speed till pale and fluffy, about 2 minutes. Add the eggs and vanilla and beat briefly to combine.
Now add the yoghurt and beat lightly. Then sift in the flour, baking powder and salt. Fold gently with a spatula to combine into a soft, thick mixture. You can use the hand mixer too, but the flour may fly around a bit.
Now pour in the liquid cocoa mixture. Stir it in, then fold in the chocolate chips (reserve a couple of tablespoons of the chips). If needed, use a hand mixer to break up any lumps in the batter.
Spoon the batter into the prepared cake tin and smoothen the top. Sprinkle with the reserved chocolate chips and bake for 45 to 50 minutes until a toothpick poked in the center comes out clean. Since ovens vary, start checking for doneness at 40 minutes. Make sure you don't mistake melted chocolate chips for wet batter! A few moist crumbs on the toothpick are ok.
Let the cake cool completely to room temperature. This also helps any cracks on the surface to close up. Then slice and dig in! Store at room temperature in an airtight tin for 2 to 3 days, although in humid weather, I prefer to store it in the fridge where it will last 10 days. Simply bring it to room temperature or lightly warm up a slice before eating. Happy baking!