Preheat oven to 425°. Line bottom of 4 (4-oz.) ramekins* or custard cups with a round of waxed paper, then grease; set aside.
Microwave chocolate in medium microwave-safe bowl at HIGH 45 seconds or until chocolate is melted. Stir in Hellmann's® or Best Foods® Real Mayonnaise until smooth. Beat in granulated sugar and flour with wire whisk until blended. Beat in eggs and vanilla. Evenly spoon into prepared ramekins. Refrigerate 1 hour or until ready to bake.
Arrange ramekins on baking sheet. Bake 13 minutes or until edges are firm but centers are still slightly soft.(DO NOT OVER BAKE.) On wire rack, cool 5 minutes. To serve, carefully run sharp knife around cake edges. Unmold onto serving plates, then remove waxed paper. Sprinkle with confectioners sugar and serve immediately.
*To bake in 12-cup muffin pan, line bottoms of 8 muffin cups with waxed paper, then grease. Evenly spoon in batter. Refrigerate as above. Bake at 425° for 9 minutes or until edges are firm but centers are still slightly soft. (DO NOT OVER BAKE.) On wire rack, cool 5 minutes. To serve, carefully run sharp knife around cake edges and gently lift out of pan. (Do not turn pan upside-down to unmold.) Arrange cakes, bottom side up, on serving plates, 2 cakes per serving. Remove waxed paper and sprinkle as above.