Preheat the oven to 350 degrees F. Prepare 4 (8-ounce) ramekins or a 1 1/2-quart souffle dish by greasing with softened butter and coating with granulated sugar, pour out any excess. The butter and sugar will keep the souffles from sticking to the sides and will allow them to rise evenly. The sugar will also give the souffle a crunchy crust, which is a great contrast to the soft interior.
To make the fruit puree base, heat the raspberry puree, 3/4 cup sugar and butter in a saucepan and place over medium heat. Cook for 10 minutes to dissolve the sugar. Remove from heat, cool slightly, then strain to remove the raspberry seeds. Whisk in the egg yolks 1 at a time.
In a separate clean bowl, beat the egg whites and cream of tartar just until they hold soft peaks. Fold 1/3 of the beaten whites into the raspberry mixture to lighten it. Then gently fold in the rest. Spoon into the prepared ramekins or souffle dish and place on a cookie sheet. Bake on the middle rack for about 20 minutes. The souffle is done when it has puffed over the rim, the outside is golden and the center giggles slightly. Take care not to over bake. Dust with powdered sugar before serving.
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