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.
Spoon half the batter into your baking dish and use your hands to press it down into the shape of the pan. Gently press 16 gluten free/vegan chocolate sandwich cookies into the batter. Cover with the rest of the batter.