Halloween Gingerbread Sandwich Cookies

To make the cookies: In a bowl, whisk flour, cocoa, spice, baking powder, and salt. In a cup, stir together molasses, water, and baking soda. In a large bowl, with mixer on medium, beat butter and sugars until fluffy. Beat in egg until blended. With mixer on low, alternately beat in flour mixture and molasses mixture until dough forms. Divide dough in half; place each half between sheets of waxed paper. Roll out to 1/8-inch thickness. Stack on baking sheet; freeze 30 minutes, until firm. Heat oven to 350 °F. Line cookie sheets with parchment. Working with 1 portion of dough at a time, peel away 1 sheet of the waxed paper to completely release from dough; pat paper back into place. Turn dough over; peel off second sheet and discard. Using 3-inch Halloween-shaped cutters, cut out 20 cookies. Transfer cookies from waxed paper to prepared cookie sheets with a small spatula, spacing them 1 1/2 inches apart. (For bat, use a skewer to cut out eyes; for jack-o'-lantern, use a small knife.) Reroll scraps; freeze. Cut out the remaining 20 bottom cookies. Bake each sheet 8 minutes. Cool on sheet 2 minutes; remove to a wire rack to finish cooling. To make the filling: Beat cream cheese and marshmallow cream until smooth. Spread a heaping tablespoon of filling on each of bottom cookies; place top cookies over filling; gently sandwich together. Decorate with icing.