Heat oven to 400°F. Butter, flour four 6-ounce custard cups.
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.
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.
Bake in 400°F oven for about 12 minutes or just until cakes are set and instant-read thermometer inserted in cakes registers 160°F.
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.