Into the cornstarch water, add the milk of choice, vegetable oil and vanilla; whisk to combine. Using a whisk, slowly mix the liquid ingredients into the dry until just combined. If it gets too thick, switch to a spatula.
Using an ice cream scoop, fill the muffin cups to the top. Smooth the batter with the back of the spoon or a small knife. Sprinkle the remaining 3 tablespoons of chocolate chips on top of the batter in each muffin cup. Gently push down on the chips to make sure they stick to the batter.