In a medium bowl, whisk the first 6 ingredients. In a large bowl using an electric mixer on medium high, beat the butter and brown sugar until light and fluffy, about 2 minutes. Beat in the molasses and egg. Add the flour mixture; beat on low until just blended. Halve the dough. Shape into disks, wrap in plastic and refrigerate until firm, about 1 hour.
Position the racks in the upper and lower thirds of the oven; preheat to 350 degrees . On a floured surface, roll out 1 dough disk 1/8 inch thick. Cut out with cookie cutters; divide between two large parchment-lined baking sheets, spacing 1 inch apart. Re-roll scraps; cut out cookies. Repeat with the remaining dough.
Bake until the edges are firm, 13 to 15 minutes. Transfer the baking sheets to racks; cool 2 minutes. Transfer the cookies to racks; let cool completely before decorating.