In a medium bowl, whisk together the milk, the cornstarch water and the pure vanilla extract until combined.
While whisking, slowly pour the cooled melted butter a little at a time into the other liquid ingredients.
Put all the batter into a piping bag or a zip top bag with the corner snipped off. Pipe the batter into the greased donut pans and fill to almost the top. It should fill 12 regular size donut wells. Smooth the tops.