Line a 9 inch square pan with aluminum foil. Spray with nonstick spray.
Stir together first 6 ingredients in a 3½ quart saucepan over medium-high heat, and cook, stirring constantly, until a candy thermometer registers 234° (soft ball stage) or for about 12 minutes.
Once mixture reaches 234°, remove from heat and quickly stir in the white chocolate, marshmallow creme, pecans (if using) and vanilla until well blended.
Pour into the prepared pan. Let stand 2 hours or until completely cool; cut fudge into squares.