Preheat oven to 350°F (180°C). Line cookie sheet with parchment paper; set aside. In a large bowl, combine flour, spice blend, baking powder, baking soda and salt; mix well. In mixer, beat butter and brown sugar medium-high speed until creamy. Add pumpkin; beat until combined. Add eggs and vanilla; beat until light and fluffy. (Mixture will be thin, like a muffin batter.) Gradually add flour mixture; beat until well blended.
Using a large cookie scoop (about 2" diameter), scoop batter onto prepared Cookie Sheet 2 in. (5 cm) apart; slightly flatten cookies with back of scoop. Press half of the walnuts into half of the cookies. Bake 16-19 minutes or until cookies spring back when lightly pressed. Slide parchment and cookies onto cooling rack; cool completely. Repeat with remaining batter and walnuts.
Using clean beaters, beat together butter, cream cheese, powdered sugar and marshmallow cream on medium-high speed in until well blended. To assemble cookies, turn the cookies without nuts bottom side up. Using a clean, large cookie scoop, scoop filling onto each cookie bottom. Top with remaining cookies with walnuts, pressing down slightly so that the filling spreads to the edge. Wrap each cookie individually with plastic wrap and refrigerate at least 30 minutes before serving.