Preheat the oven to 350°F (180°C).
Fill a 12-muffin tin with paper cases.

In a large saucepan, whisk together the cornstarch, cocoa, sugar, and water, being careful not to create lumps.
Bring to a boil over medium heat, whisking constantly until thickened.
Remove from heat, add the butter and chocolate, and mix until the butter and chocolate are completely melted and the mixture is smooth.

Add the oil, vanilla extract, and one egg. Mix until smooth.
Add the second egg and sugar, and beat until incorporated: the batter should thicken and remain smooth.
Incorporate the flour and baking powder, which have been sifted together beforehand.
Mix to combine everything and obtain a smooth batter.
Fill the muffin cases 2/3 full and bake for about 25 minutes.

Let cool for 5 minutes in the tin before transferring to a cooling rack.