Preheat the oven to 375°F.
Grease four ramekins with softened butter and dust with granulated sugar, shaking out the excess.
In a small saucepan over medium heat, combine the raspberries, 1/4 cup of granulated sugar, and lemon juice. Cook for about 5 minutes, mashing the raspberries with a fork until they break down and the mixture thickens. Remove from heat and let cool slightly.
In a mixing bowl, beat the egg whites and cream of tartar with an electric mixer on medium speed until soft peaks form.
Gradually add the remaining 1/4 cup of granulated sugar, continuing to beat until stiff peaks form.
Gently fold the raspberry mixture and vanilla extract into the egg whites until just combined.
Spoon the soufflé mixture into the prepared ramekins, filling them about three-quarters full.
Place the ramekins on a baking sheet and bake for 12-15 minutes, or until the soufflés are puffed and lightly golden.
Remove from the oven and dust with powdered sugar before serving.