Preheat oven to 350ºF. Line baking sheets with parchment paper.
In a stand mixer, beat together the butter and brown sugar until light and creamy, about 2 minutes. Beat in the egg, then the molasses and buttermilk, pausing occasionally to scrape down sides of bowl.
In a separate bowl, whisk (or sift) together the flour, baking powder, baking soda, salt and spices. Add to the molasses mixture and beat on medium speed until combined.
Drop by teaspoonfuls 3" apart onto prepared baking sheets. Bake for about 12 minutes, until they've lost their sheen and have a slight bounce when gently pressed.
To make the icing, stir all ingredients together in a medium bowl (I use my hands to work the butter into the confectioners' sugar). Use a spoon to frost the cookies while they're still slightly warm for a glaze effect. Sprinkle with cinnamon, if desired.