Lightly brush four 8-ounce ramekins with butter, then dust each with sugar. (This can be done up to 2 hours ahead.)
Melt the butter in a heavy medium saucepan over medium-low heat, then cool to warm. Whisk in the sugar and lemon juice and zest until the sugar is dissolved. Whisk in the egg yolks. Cook over medium-low heat, whisking constantly, until the mixture thickens and an instant-read thermometer registers 175 degrees F, about 10 minutes. Do not boil. Transfer the egg yolk mixture to a large bowl and whisk in the flour until smooth. Cool to room temperature, about 30 minutes.
Place an oven rack in the center of the oven; preheat the oven to 400 degrees F.
Put the egg whites, salt and cream of tartar if using in a large, clean nonreactive bowl. Beat the egg whites with an electric mixer until they hold soft peaks. Fold a quarter of the beaten egg whites into the egg yolk mixture to lighten. Gently fold in the remaining egg whites.
Divide the souffle mixture evenly among the prepared ramekins. Place the filled ramekins in a roasting pan. Carefully fill the pan with enough hot water to come halfway up the sides of the ramekins. Bake the souffles until golden brown on top, 12 to 14 minutes. Using tongs as an aid, carefully remove the hot souffles from the water to a plate.
If desired, make a small indentation in the center of each souffle and place a small scoop of raspberry sorbet in the indentations. Serve the souffles immediately.
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