It helps to start this cake one day before you plan to cut into it. I baked the cake in the morning and while it cooled and then chilled overnight (making it easier to slice and assemble), I made the filling, and let that chill overnight as well. The next day, I just had to put it all together and it gives the cake enough time to 'set', and then come back to room temperature before eating for the best texture.
Make the cake. Melt the chocolate in the microwave (10-second increments, stirring in between) or in a heatproof bowl over a pan of simmering water, and set aside to cool a little.
Grease a 9" springform tin and preheat the oven to 175 C.
In a large mixing bowl, beat the butter and sugar with a hand mixer till pale and light. Add the yoghurt and beat gently to combine.
Add the vanilla and the eggs, one at a time, beating gently after each addition. Now stir in the melted chocolate by hand, taking care not to be too vigorous, and mixing until just incorporated.
Sift in the flour, cocoa, baking soda and salt (if using). Fold it all in very gently till only a few streaks of flour remain. Now add the hot water, a little at a time, to prevent any lumps forming, and continue to fold the batter with a silicone spatula. I prefer doing this step by hand as a mixer could cause the hot water to splash here and there and make a mess.
If you still see lumps (I often do, despite my best efforts), beat with the hand mixer on low speed for a few seconds until smooth. The batter is slightly runny.
Pour the batter into the prepared loaf tin and bake for 50 minutes to 1 hour, until a toothpick poked in the center comes out clean. If you feel the top is darkening too fast, cover very loosely with foil and continue baking. Because ovens vary, it's a good idea to check around 45 minutes and then add more baking time as needed, but avoid opening the oven before this.
Once baked, let the cake cool COMPLETELY at room temperature. Do not attempt to take it out of the tin while it's warm as it's definitely fragile. Once cooled, loosely cover the tin and place it in the fridge to chill overnight (or at least 6 hours) before assembling.
Make the filling. Preheat the oven to 190 C. Spread the pecans out in a single layer on an un-greased baking tray. Bake for 5 minutes until toasty and fragrant. Transfer to a plate and when cool, break the pecans up into coarse pieces. Lower the oven temperature to 175 C, then spread the coconut out onto the same tray. Bake for 15 minutes, stirring the coconut at least 4 times as it begins to brown and dry up, so that you make sure it's not sticking and is getting evenly toasted. Set aside to cool completely.
Now in a medium sized saucepan or pot (a deeper one is better as the filling needs some room to simmer), whisk together the cornflour and two tablespoons of the milk, until smooth.
Pour in the rest of the milk and cream, then add the sugars and butter. Give it a quick whisk to combine, then place on low heat. Use a silicone spatula or a whisk to stir slowly and continuously. Let the butter melt and as the mixture cooks and thicken up, it will begin to simmer. It should thicken enough to coat the back of a spoon (see below). The whole thing takes 5 to 7 minutes so make sure you're stirring constantly to prevent burning or sticking. Take the pan off the heat and let the custard cool for about 10 minutes at room temperature where it will continue to thicken. Finally, stir in the toasted pecans and coconut (I reserved 3 tablespoons of the toasted coconut to decorate the top of the cake but it's totally optional.) Let the prepared filling cool for another 30 minutes (it will now be quite thick), then loosely cover and place in the fridge overnight (or at least 6 hours).
The next day, you'll see that the filling has taken on the consistency of cooked oats. Let it sit at room temperature for about 30 minutes and it will loosen up enough to be of a spreadable consistency.
Take the chilled cake out of the fridge. Remove the sides of the springform pan (I assembled the cake on the base of the pan itself, but you can transfer it to a platter if you like). The cake rises quite flat, but if you do have a slight dome, level it off with a sharp knife. Then use a long, serrated knife to slice the cake in two horizontally. I place my hand gently on the top of the cake and rotate it slowly to get an even cut. Lift off the top with a large metal spatula and place on a plate nearby.
Drop half the filling into the center of the lower half of the cake. Spread it evenly right to the edge of the cake. Gently place the top half over it, and spread the rest of the filling on top as well. Scatter the reserved toasted coconut (optional) on top and if you're feeling enthusiastic, grate a couple of squares of dark chocolate over it as well! This cake is usually left with unfrosted sides but some recipes do add a chocolate buttercream frosting or ganache if you're interested!
Let the cake sit at room temperature to 'set' for about an hour. If it's very warm where you are, then place it in the fridge to set. I cut the cake while it was at room temperature itself because I like the softer texture, but if you're looking for thinner, very neat slices, then chilling is recommended.
Store in the fridge at all times, where it will keep for 4 to 5 days, and in the freezer for about a month. Happy baking!