Cream the butter, oil, sugar, vanilla and salt in the bowl of a stand mixer fitted with a paddle attachment, just until the mixture is smooth. (Do not over beat, as this will incorporate air into the dough, which may lead to spreading.)
Add the egg and mix just until incorporated.
Mix in the cornstarch, cocoa powder and 1 1/4 cups of the flour.
Continue adding flour until the dough gathers itself into a ball and pulls away cleanly from the sides of the bowl. (You may not need all the flour.)