Preheat oven to 375°. Cream butter and brown sugar until light and fluffy. Beat in eggs, molasses and vanilla. In another bowl, whisk together flour, baking soda, salt and spices; add to creamed mixture alternately with buttermilk, beating well.
Drop dough by tablespoonfuls 2 in. apart onto greased baking sheets. Bake until set, 7-9 minutes. Remove to wire racks; cool completely.
For frosting, beat butter, shortening and ginger until light and fluffy; beat in vanilla. Gradually beat in confectioners' sugar and milk. Spread over cookies. If desired, decorate with sprinkles. Store in airtight containers.
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