Billed as Paris’ first boutique devoted entirely to butter, Le Butter Shop opened last month a short walk from the Eiffel Tower – with 30 varieties from small family producers in Brittany, the Loire Valley and northern France. France eats around 8kg (17.6 pounds) of butter per person a year, according to the country’s Ministry of Agriculture and Food. It is among the highest in the world. Yet the obesity rate is well below the average for wealthy nations – 14 per cent, compared to an average of...