Grease a 22cm (or slightly larger) microwave safe silicone cake pan with sunflower oil and place a circle of baking parchment in the bottom (the cake also works without the baking parchment).
Mix sugar, flour, cocoa and baking powder in a bowl. In a separate bowl, whisk the oil, eggs, vanilla extract and 100ml hot water until combined.
Add liquid ingredients to the dry ingredients and mix thoroughly until you have a lump free mix. Pour mixture into the cake pan and gently tap to pop any air bubbles. Cover with cling film.
Microwave on full power for 7 minutes (800w) check if the cake is ready using a skewer. If the skewer comes out clean then the cake is ready. Once ready, leave to stand for a few minutes and then remove cling film and transfer to a cooling rack.
Put the chocolate into a microwave safe bowl and melt in microwave for approximately 2 minutes, stirring every 30 seconds until melted. Add the double cream and mix thoroughly until smooth and creamy.
Place the cake on a serving plate then drizzle with the chocolate ganache. Slice and serve.
I recommend topping with chocolate sprinkles or something like that for a better taste.