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.
Once out of the oven, let the muffins cool in the pan for about 5-10 minutes. Then remove from the pan using a small spatula and let cool on a wire cooling rack.