Preheat the oven to 375 degrees F. Grease each cup of the popover tin with a teaspoon of the butter. In a mixing bowl, whisk the eggs until frothy. Whisk in the milk and remaining butter. In a mixing bowl, combine the flour and salt together. Whisk the egg mixture into the flour mixture and mix well. Pour the batter into 10 of the prepared popover tins. Bake the popovers for about 50 minutes, undisturbed. Remove from the oven and serve hot with jam.