1. Preheat oven to 350°. Line 1 (12-cup) muffin pan with paper baking cups.
2. Beat butter and sugar in a large bowl at high speed with an electric mixer until fluffy. Add eggs, 1 at a time, beating well after each addition. Beat in vanilla.
3. Combine flour and next 3 ingredients. Stir flour mixture into butter mixture, alternating with milk and beginning and ending with flour. Fill muffin cups evenly with batter.
4. Bake 20 to 25 minutes or until a toothpick inserted in center comes out clean. Transfer to wire racks to cool completely.
5. Meanwhile, heat cream in a small saucepan over low heat until hot. Remove from heat, and stir in chocolate and peppermint extract until mixture is smooth and well blended. Let cool completely.
6. Core cupcakes with cupcake corer. Pipe or spoon about 2 teaspoons chocolate mixture into center. Pipe or spoon Crème de Menthe Buttercream on top. Garnish, if desired.