Preheat oven to 375F. Place 24 paper muffin cup liners in muffin cups.
To prepare cupcakes, sift together flour and next 4 ingredients (flour through cinnamon). Place butter and brown sugar in a large bowl; beat with a mixer at medium speed until light and fluffy. Add eggs, 1 at a time, beating well after each addition. Add pumpkin, beating on low speed. Add flour mixture and milk alternately to sugar mixture, beginning and ending with flour, beating on low speed. Stir in nuts.
Spoon batter into muffin cups, filling each about two-thirds full. Bake 25 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes in pan on wire rack; remove from pan. Cool completely on wire rack.
To prepare icing, place all ingredients in large mixing bowl; beat with mixer on medium speed until smooth. Spread icing onto top of cooled cupcakes.