Put a metal or Pyrex pie plate or cake pan in the freezer to chill. Lightly butter six 6-oz. ramekins or custard cups. Coat with sugar and tap out the excess.
Make the sauce:
Purée the raspberries in a food processor. Transfer the purée to a fine sieve set over a small bowl. Strain the purée by pressing and scraping with a rubber spatula. Discard the seeds.
In a medium heatproof bowl set in or over a skillet of barely simmering water, combine the chocolate, butter, sugar, and 2 Tbs. of the raspberry purée (save any extra for another use). Stir frequently with a rubber spatula until melted and smooth. Scrape into a puddle on the chilled plate and return to the freezer until firm, 20 to 30 min. When the raspberry-chocolate mixture is firm, use a teaspoon to scrape it into six rough balls. Keep the balls on the plate and refrigerate until ready to use.
Make the soufflé cakes:
Put the 3 egg yolks in a medium bowl near the stove and have another large, clean bowl at hand. Combine the flour and salt in a small, heavy saucepan. Whisk in just enough of the milk to make a smooth paste. Whisk in the remaining milk. Set the pan over medium heat and cook, whisking constantly, until the mixture has the consistency of a thick cream sauce, 2 to 3 min. Whisk about 2 tablespoons of the flour-milk mixture into the yolks to warm them up gently. Scrape the yolks back into the saucepan and cook for a minute or two, whisking constantly, until the mixture becomes a thick pastry cream; it should be about as thick as store-bought mayonnaise. Use a rubber spatula to scrape the pastry cream into the clean bowl. Add the white chocolate and whisk until it’s fully melted and incorporated into the warm pastry cream. Stir in the vanilla. Set aside for a few minutes until tepid.
In a clean, dry bowl, beat the egg whites and cream of tartar on medium speed in a stand mixer (or on high with a hand-held mixer) until the whites mound gently. Gradually beat in the sugar and beat until the whites form medium-stiff peaks when you lift the beaters; the tips should curl over but still look moist, glossy, and flexible. With a rubber spatula, fold about one-quarter of the whites into the white chocolate pastry cream to lighten it. Scrape the remaining whites into the bowl and gently fold in until blended, taking care not to deflate the whites. Take the chocolate balls out of the refrigerator and put one ball in the center of each ramekin. Divide the batter evenly among the ramekins and level the tops gently with the back of a spoon.
Position a rack in the lower third of the oven and heat the oven to 375°F. Put the ramekins on a baking sheet. Bake until the cakes are puffed and golden brown on top—they’ll quiver when tapped and seem soft in the center, 16 to 18 min. Let cool for a few minutes before serving.
Make Ahead Tips
Assembled, unbaked soufflé cakes can be covered with plastic wrap and refrigerated for up to two days.
Try replacing the raspberry purée with orange juice and orange liqueur, plus a little grated orange zest.
Photo: Scott Phillips