1In small bowl, mix brown sugar, cocoa, chili powder and cinnamon. Pat mixture on both sides of chicken; shake off any excess.
2Heat grill pan or 10-inch skillet over medium-high heat. Add chicken; cook 8 to 10 minutes or until juice of chicken is clear when center of thickest part is cut (165°F). Remove chicken from pan; cool 5 minutes.
3Meanwhile, in small bowl, beat vinegar, oil and honey with whisk until well blended. Add endive and radicchio; toss to lightly coat.
4Place each tostada shell on individual serving plate. Shred chicken; divide evenly onto shells. Top each with endive-radicchio mixture and sour cream. Garnish with chocolate shavings.