In a small, heavy bottomed saucepan, melt chocolate and butter over low heat. Let chocolate mixture cool. (Alternatively, chocolate and butter may be melted in the microwave. Be careful not to overheat and burn it.)
In a mixer, with the balloon whip attachment, whisk eggs and sugar until thickened and lemon-colored; blend in vanilla. Sift flour and salt together and fold in; blend in chocolate mixture on low speed. Mix until thoroughly blended.
Prepare ramekins by greasing with butter and dusting of flour mixed with some cocoa powder.
Scoop batter into prepared ramekins and push truffle into center of batter. Bake at 400F for 18-20 minutes or until the edges are cooked and the middle is just set. Garnish with sprinkle of powdered sugar.
Variation: For Espresso flavor, add 1 teaspoon instant coffee powder to chocolate-butter mixture as it melts.