Position your oven rack to the lower third of your oven. Then preheat the oven to 375 degrees.
Grease eight (6 oz.) ramekins with the softened butter. Coat the inside with granulated sugar and gently tap out an excess.
In a large mixing bowl, beat egg whites at high speed until foamy, about 30 seconds. Add cream of tartar and beat for an additional 30-60 seconds or until very soft peaks form. Add the confectioners’ sugar, one tablespoon at a time, and beat until stiff peaks form, about 30 seconds more.
Stir the room-temperature meringue with a spatula to loosen it. Gently add a third of the meringue to the pastry cream bowl. Gently run the spatula around the edge of the bowl, then bring it through the middle. Continue “folding” the mixture until it is mostly incorporated, however there will be a few streaks left. Continue this process with the remaining pastry cream – one third at a time – until all of the meringue is incorporated and and there are no white streaks remaining.
Fill the prepared ramekins with the soufflé mixture, being sure to smooth the tops with an offset spatula. Run your index finger the edge of each ramekins to create a small trench around the edge of the soufflé (this will help the soufflé rise). At this point you can store the prepared soufflés in the refrigerator for up to two hours.
Place the ramekins on a rimmed baking sheet and gently place them in the oven. Bake for about 15-20 minutes, or until a toothpick inserted near the center comes out with just the tip wet.
Quicky dust with confectioners’ sugar and serve immediately.