Put the milk, sunflower oil, vanilla, flour, caster sugar, cocoa powder and baking powder into a jug and whisk for 2 mins until well combined. Set aside.
Line a microwave-safe bowl (about 12cm deep and 12cm wide) with clingfilm, ensuring it hangs over the side by at least 1cm.
Pour one-third of the batter into the bowl and microwave on high* for 1 min 15 secs. Set aside for 1 min.
Pour the rest of the batter into the bowl, then spoon the chocolate spread on top. Microwave on high for 2 mins 45 secs, then leave to stand for 2 mins.
Place a plate over the bowl and carefully invert the bowl onto the plate. Tap the bowl and carefully remove with the clingfilm. Dust the cake with the icing sugar and serve with a scoop of vanilla ice cream, if you like.