Preheat the oven to 180 C / Gas 4. Line a 12-hole muffin tin with paper cases.
First make the custard: put the cornflour, cocoa, brown sugar, and water into a saucepan and whisk together over a medium heat until boiling, very thick and smooth. Remove from the heat immediately, beat in the butter and chocolate until melted and absorbed, then add the oil, vanilla and one of the eggs and beat until combined. Add the remaining egg and the caster sugar, and beat again until smooth and thick.
Measure the plain flour and baking powder into a bowl, stir together, then sift directly into the custard and beat until combined. Spoon into the prepared paper cases, filling only to 3/4 full to ensure a good rise.
Bake in the preheated oven for 25 minutes, or until a skewer inserted into the centre comes out clean. Remove from the oven and allow to cool before decorating or serving.
This recipe takes about 40-45 minutes. You can also add chocolate chips to the recipe for a yummy melting sensation!