Line a 9-in. square pan with foil. Grease the foil with butter, set aside.
In a large heavy saucepan, combine the 1/4 cup butter, sugar and sour cream. Cook and stir over medium heat until sugar is dissolved. Bring to a rapid boil, cook and stir until a candy thermometer reads 234° (soft-ball stage), about 5 minutes.
Remove from the heat, stir in white chocolate and marshmallow cream until melted. Fold in peppermint candy and extract.
Immediately pour into the prepared pan. Sprinkle with more crushed peppermint if desired. Chill until firm.
Use the foil to lift the fudge out of pan. Gently peel off foil, cut fudge into 1-inch squares. Store in the refrigerator.