In large bowl, beat 1 cup butter and egg yolk with electric mixer on medium speed, or mix with spoon. Stir in flour, cornstarch, 1/2 cup powdered sugar, the cocoa and salt. Cover; refrigerate about 1 hour or until firm.
Heat oven to 375°F. Shape dough into 1-inch balls. On ungreased cookie sheet, place balls about 2 inches apart.
Bake 10 to 12 minutes or until set but not brown. Remove from cookie sheet to wire rack. Cool completely, about 30 minutes.
In small bowl, mix all frosting ingredients except candy canes with spoon until smooth and spreadable. Frost cookies; sprinkle with crushed candy canes.