Soft and sugary, it has spurred the spirits of World War One soldiers on the front lines. Warm and doughy, it has earned the adulation of Homer Simpson, the main character of America's longest-running animated TV series. Heart-shaped and jelly-filled, it has even been used to honour the Duke and Duchess of Sussex's wedding. And every 2 June since 1938, we celebrate it - the donut or doughnut - on National Donut Day in the US