Position a rack in the lower third of the oven and move (or remove) the other rack so there is at least 6 inches of space above the first rack. Heat the oven to 425°F. Generously butter six 6-oz. ramekins and coat the buttered surfaces lightly but thoroughly with sugar.In a large bowl, whisk the marmalade, lemon juice, orange liqueur, and salt.
In a large, deep bowl, beat the egg whites with an electric mixer on medium speed until foamy, 30 seconds. Add the cream of tartar, increase the speed to medium high, and beat until the egg whites form soft peaks, about 1 minute. Gradually add the sugar, about 1 Tbs. at a time, and continue to beat just until the whites form stiff peaks, 1 to 2 minutes.
With a rubber spatula, fold a third of the egg whites into the marmalade mixture. When incorporated, fold in another third, and then the remaining third. Fill the ramekins halfway with the soufflé mixture and transfer to a baking sheet. Bake until puffed and dark brown on top, 12 to 15 minutes. Serve immediately.
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