Preparation

1. Preheat the oven to 350°F. Line baking sheets with parchment paper or foil.
2. Using an electric mixer, cream the butter and sugar until light and fluffy. Beat in the egg, vanilla, cinnamon, ginger, and baking soda, scraping the bowl as needed.
3. In a separate small bowl, whisk together the molasses and the sour cream.
4. With the mixer on low speed, add the flour and the molasses mixture to the batter in several alternating additions, beginning and ending with flour.
5. Scrape the bowl and beat a few seconds more to ensure the ingredients are well distributed.
6. Drop the batter by spoonfuls on the baking sheets, leaving space in between for the cookies to spread.
7. Bake for eight minutes, until the cookies are barely firm on the edges and still quite soft in the middle.
8. Cool briefly on the pans, then transfer to racks to cool completely.
9. Whisk together the powdered sugar and enough water to form a spreadable glaze. Stir in the lemon zest.
10. Frost the cookies and set them aside for about one hour while the glaze sets. Store at room temperature for up to four days.
11. For best results, keep the cookies in an airtight container separated by layers of waxed paper.
12. (The unfrosted cookies can be baked ahead and frozen for up to three months, then thawed and glazed as needed.)