1. Heat oven to 400 degrees F. Butter, flour four 6-ounce custard cups.
2. Mix cream and espresso in small saucepan. Bring to simmering over medium-low heat, stirring to dissolve powder. Add butter and chocolate; stir until melted and smooth. Remove from heat. Stir in vanilla and flour.
3. Beat eggs and sugar in bowl on high until light and fluffy, 2 minutes. Stir in chocolate mixture. Pour into cups. Place in 9-inch square pan.
4. Bake in 400 degrees F oven for about 12 minutes or just until cakes are set and instant-read thermometer inserted in cakes registers 160 degrees F.
5. With oven mitts, remove pan from oven. Remove cups from pan; let stand 1 minute. Run small spatula around side of each cup; invert cakes onto individual dessert plates; remove cups. (Unmold immediately or cakes will stick.) Leave cups over cakes to keep warm, if desired. Dust with confectioners sugar and serve with vanilla ice cream, if desired.