Line a 1/4 sheet pan with parchment paper; grease paper. Set aside. Bloom gelatin in 4 oz. water; set aside.
Meanwhile, combine granulated sugar, corn syrup, remaining 3 oz. water and honey in heavy saucepan. Bring to boil on medium heat. Reduce heat to medium-low. Cook until mixture reaches 252°F, stirring just until sugar is dissolved. Remove from heat and pour into bowl of mixer fitted with whip attachment. Cool, without stirring, to 212°F.
Heat gelatin mixture over hot water bath until dissolved. Add to sugar mixture in mixer bowl. Whip 8 min. Fold in vanilla and OREO Pieces.
Spread mixture evenly into prepared pan. Place a second piece of greased parchment paper over marshmallow mixture. Let stand at room temperature at least 8 hours or overnight.
When ready to cut, remove top piece of parchment paper. Invert onto cutting board; remove second piece of parchment. Combine cornstarch and powdered sugar; sift a portion over top of marshmallows. Trim sides, then cut into 54 pieces in 6x9 pattern. Sift additional cornstarch mixture over cut marshmallows. Shake marshmallows in a strainer to remove excess cornstarch. Store in airtight container at room temperature.