What was the first food to be deliberately microwaved?

Popcorn was actually the first food to be microwaved deliberately. In 1946 Dr. Percy Spencer of the Raytheon Corporation was experimenting with a magnetron (a new type of vacuum tube) when he noticed that the chocolate candy bar in his pocket had melted. He was curious, so he placed some popcorn kernels next to the magnetron and turned it on - the popcorn popped.

After experimenting with various other food items (including an egg that exploded!), he and Ratheon realized they were on to something and continued on to develop the first microwave oven.

Microwave ovens are a common kitchen appliance and are popular for reheating previously cooked foods and cooking a variety of foods. They are also useful for rapid heating of otherwise slowly prepared foodstuffs, which can easily burn or turn lumpy when cooked in conventional pans, such as hot butter, fats, chocolate or porridge.