Grease a 9" x 5" x 3" loafpan. Line pan with plastic wrap, making it smooth in pan and allowing it to extend slightly over edges of pan. Set aside.
Melt butter in a large skillet over medium heat; add pecans. Cook, stirring constantly, until pecans are toasted. Remove from heat; let cool completely.
Place chopped white chocolate in a small bowl. Bring milk to a simmer in a small heavy saucepan over medium-low heat. (Do not boil.) Remove from heat, and pour over chopped chocolate, stirring until chocolate melts. Let cool completely.
Press chopped cherries between paper towels to remove excess moisture. Chop frozen candy bars.
Beat whipping cream at high speed of an electric mixer until soft peaks form. Gradually add sugar, beating until blended. Gently fold in melted white chocolate mixture, chopped candy bars, and pecans; then fold in cherries.
Spoon mixture into prepared pan; cover and freeze 24 hours. To serve, dip pan in warm water 30 seconds. Invert loafpan onto a serving plate. Peel off plastic wrap. Cut terrine into 3/4" slices. Drizzle fudge topping on each plate, if desired; top with a slice of frozen terrine. Garnish, if desired.
Note: For chocolate-coated coconut bars, we used Mounds.