Chocolate-Banana Souffles

2 ounces semisweet chocolate, coarsely chopped
1/2 cup mashed ripe banana, (1 large)
2 teaspoons instant espresso powder, or instant coffee
3 tablespoons sugar, plus extra for preparing souffle dishes
2 teaspoons vanilla extract
1/3 cup unsweetened cocoa powder, preferably Dutch-process
3 tablespoons cornstarch
1/4 teaspoon salt
1/4 cup packed light brown sugar
1/4 cup low-fat milk
5 large egg whites
1/8 teaspoon cream of tartar
Confectioners' sugar, for dusting

Preheat oven to 350°F. Lightly oil six 6-ounce (3/4-cup) individual souffle dishes or ramekins or coat them with nonstick spray. Dust with sugar, turning to coat, and tapping out the excess. Set aside.

Melt chocolate over barely simmering water or in the microwave. Dissolve espresso powder (or instant coffee) in vanilla in a small bowl.

Sift cocoa, cornstarch and salt into a large bowl. Add banana, brown sugar and milk, whisking to blend. Add melted chocolate and the coffee mixture and blend well.

Place egg whites in a clean, grease-free mixing bowl. Beat on low speed with an electric mixer until frothy. Add cream of tartar; increase the speed to medium and whip until soft peaks form. Gradually add the remaining 3 tablespoons sugar and beat on medium-high until the whites hold stiff, but not dry, peaks.

Whisk one-fourth of the egg whites into chocolate mixture. Gently but quickly, fold in the remaining whites with a rubber spatula. Spoon into the prepared souffle dishes or ramekins.

Bake until souffles are well puffed and browned, but still jiggle slightly when shaken, 15 to 25 minutes, or. Remove from the oven and sift a little confectioners' sugar over the top. Serve immediately.