Place the chopped chocolate in a microwave safe bowl. Add the butter and vegetable oil to the bowl. Heat in 15 second increments and stir after each time. Repeat until the chocolate is completely melted and silky smooth.
Into the melted chocolate and butter, mix in the cornstarch water until fully combined. Into the chocolate mixture, whisk in the sugar, vanilla, baking powder and salt until combined. Then whisk in the milk vinegar mixture until combined and smooth.
Bake the brownies in the center rack of the oven at 350 degrees Fahrenheit for 40 minutes. Let the brownies cool almost completely in the pan before trying to remove them, otherwise they will break apart into a giant mess.