Pre-heat your oven to 170c/325f, and also grease and line an 8 inch square cake tin.

In a bowl or jug, mix together the cocoa powder, the hot water mixed with coffee and the oil. Then mix in the yoghurt and eggs.

In a separate large bowl, whisk together the flour and both sugars.

Now add the wet mixture into this bowl and mix everything together just until you get a smooth batter.

Pour this into your tin and bake for around 45-55 minutes, or until a toothpick inserted into the centre comes out clean.

In a pot, add in the egg yolks, granulated sugar, salt, corn flour and milk. Mix all this together and then place the pot over medium-low heat.

You want to keep heating this, whiles constantly stirring, until the mixture thickens. Try not to increase the heat or stop stirring during the process, if the egg yolks get too hot, they will scramble.

Once the mixture thickens, take it off the heat, then add in the butter and chocolate.

Stir until melted, then push this through a sieve to get any lumps out.

Cover with cling film until you're ready to use it. Make sure that the cling film is touching the top of the custard, so that a skin doesn't form.

Once your cake is baked and your custard is made, you can eat it.

Unlike most cakes, you don't need to wait for this one to completely cool. Just take it out of the tin, cut into it, pour some of the warm custard on top and enjoy!