Position a rack in the lower part of the oven, leaving plenty of room above for the soufflés to rise. Preheat the oven to 375°F.

With a pastry brush or your hands, generously butter the insides of six (8 ounce) souffle ramekins. Add some granulated sugar to each ramekin, and swirl and turn the ramekin to evenly coat the interior with sugar. Place the ramekins on a sheet pan.

In a blender, puree the strawberries with the ½ cup granulated sugar. Add the vanilla. Process until very smooth, then scrape the puree into a bowl with a silicone spatula.

In the bowl of a stand mixer fitted with a whisk attachment, begin to whip the egg whites and cream of tartar on medium speed. Gradually add the powdered sugar. Continue to whip until the meringue is very firm, stands up straight when the whisk is lifted, and the appearance is shiny. This could take 5 to 10 minutes or longer, depending on the temperature of the egg whites.

Remove the bowl from the mixer. With the spatula, gently fold half of the strawberry puree into the egg whites. (Reserve the other half for serving). Divide the strawberry meringue among the prepared ramekins; they should be slightly overfilled. Lightly tap each ramekin against the counter. Decoratively place strawberry slices on top of the souffles.

Place the sheet pan in the oven and bake the soufflés for about 15 minutes, or until very puffy and set. Dust with powdered sugar and serve immediately with the remaining strawberry puree.