The answer is:
one batch of normal cookies recipe
one with one ingredient changed
Controlled experiment refers to the type of experiment when the researcher had the power to either include or isolate a variable that can influence the result of the experiment.
By doing two different batch with normal recipes and changed ingredient, the baker fully know what variable that caused the difference in each batch. He can attribute all the difference in result to that variable and might be able to reproduce the result in the future.