That brings to mind:

https://en.wikipedia.org/wiki/Military_chocolate_(United_States)

The first chocolate ration bar commissioned by the United States Army was the Field Ration D or Ration, Type D, commonly known as the "D ration." Army Quartermaster Colonel Paul Logan approached Hershey's Chocolate in April 1937, and met with William Murrie, the company president, and Sam Hinkle, the chief chemist. Milton Hershey was interested in the project when he was informed of the proposal, and the meeting began the first experimental production of the D ration bar.

Colonel Logan had four requirements for the D ration bar, dictating that it had to:

• Weigh 4 ounces (113.4 g)
• Be high in food energy value
• Be able to withstand high temperatures
• Taste "a little better than a boiled potato" (to keep soldiers from eating their emergency rations in non-emergency situations)

Hinkle was forced to develop entirely new production methods to produce the bars. Each four-ounce portion had to be kneaded, weighed, and pressed into a mold by hand. The end result was an extremely hard block of dark brown chocolate that would crumble with some effort and was heat-resistant to 120 °F (49 °C).

After U.S. entry into the Second World War, Congress planned to shut down the candy industry for the duration of the conflict, deeming it non-essential. Milton Hershey, fighting off attempts to ration sugar, corn syrup, and cocoa, claimed to Congress that chocolate was a vital source of nutrition for the nation's troops.[5] During the war years, the bulk of the Hershey Food Corporation's chocolate production was for the military. Between 1940 and 1945, an estimated three billion units of the specially formulated candy bars were distributed to soldiers around the world."[6]

The D ration was almost universally detested for its bitter taste by U.S. troops, and was often discarded instead of consumed when issued.[7] Troops called the D ration "Hitler's Secret Weapon" for its effect on soldiers' intestinal tracts.[7] It could not be eaten at all by soldiers with poor dentition, and even those with good teeth often found it necessary to first shave slices off the bar with a knife before consuming.[7]