Heat oven to 350°F. Grease and flour 12-cup fluted tube cake pan, or spray with baking spray with flour.
For filling, in a medium microwavable bowl, microwave chocolate chips, milk and peppermint extract uncovered for 30-40 seconds; stirring until melted and smooth. Set aside.
In large bowl, beat cake mix, sour cream, oil and eggs with an electric mixer on low speed for 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Spoon half of batter into pan; spread evenly. Bake for 15 minutes.
Remove from oven; immediately drop spoonfuls of filling over batter, keeping filling away from sides of pan and pressing down lightly. Carefully spoon remaining batter over top; spread evenly. Bake 42-47 minutes longer or until toothpick inserted near center comes out almost clean and cake springs back when lightly touched in center.
Cool in pan 15 minutes.
Loosen cake with knife around center and edges of pan. Turn cake upside down onto cooling rack or heatproof serving plate. Cool completely, approximately 1 1/2 hours.
For frosting, in a medium saucepan, add butter, milk and sugar. Bring to boil and boil for 30 seconds. Remove from heat and add chocolate chips, stirring constantly until melted. Pour immediately over top of cake and garnish with crushed peppermint candies.
