Preheat oven to 350 degrees F. Line heavy baking sheet with parchment paper; butter paper. Generously butter eight 3-inch-diameter ring molds with 2-inch-high sides. Arrange molds on prepared baking sheet. Stir chocolate and butter in large wide metal bowl set over saucepan of simmering water until smooth. Remove from heat. Cool 10 minutes, stirring occasionally.

Using an electric stand mixer and whisk attachment, beat sugar, eggs, yolks, 2 teaspoons espresso powder, vanilla and salt in large bowl until very thick ribbon falls when attachment is lifted, about 8 minutes. Sift flour over egg mixture, then fold in. Fold egg mixture into cooled chocolate mixture. Divide half of batter among molds. Spoon 1/4 teaspoon espresso powder into center of each. Top with remaining batter.

Bake cakes until tops are puffed and dry and tester inserted into center comes out with moist batter still attached, about 12 minutes. Cool cakes 5 minutes.

Using a metal spatula, transfer cakes and molds to plates. Carefully remove molds. Sift powdered sugar over cakes and serve immediately.