Heat oven to 350°F. Line muffin cups (2-1/2 inches in diameter) with paper bake cups.
Stir together flour, sugar, baking soda and salt in large bowl. Add water, oil, vinegar and vanilla; beat with whisk or spoon until batter is smooth. Blend in melted chocolate. Fill muffin cups 2/3 full with batter.
Bake 16 to 18 minutes or until wooden pick inserted in center of cupcakes comes out clean. Remove from pan to wire rack. Cool completely. Frost cupcakes with Butter cream frosting. Place a Junior Mint in center of the cupcake. Place 4 M&Ms around the top of the mint patty to complete the 'paw print' 12 cupcakes.
In a large bowl, cream butter and shortening until light and fluffy. Add salt, vanilla, confectioners' sugar and milk. Beat well.