Make the cookies

1. Preheat the oven to 325 F (165 F).  Line two sheet trays with parchment

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   paper.
2. Mix the butter, sugar, molasses, gingerbread spice, baking powder, baking soda, salt, and lemon zest in either a stand mixer or with a hand mixer until well-blended.
3. Scrape down the sides of the bowl, and then add in the rum, egg, and sour cream.  Mix until the mixture is smooth.
4. Add in the flour and mix until the flour just disappears.  Scrape down the bowl to mix in any butter or molasses that might be hanging out in the bottom of the bowl. You will have a very sticky batter. 
5. Scoop out balls of dough with a cookie scoop, 12 to a sheet tray.  Lightly dampen your hands and gently flatten the balls just to smooth the tops.
6. Bake the cookies for 16 minutes, rotating the sheets top to bottom and turning them 180 degrees around halfway through baking.  The cookies are done when they are dry on top, and the middle of the cookies does not collapse when you touch them.

Finish the cookies

1. While the cookies are baking, beat all the royal icing ingredients together until the icing holds its shape and is thick and bright white.  Scrape the icing into a piping bag fitted with a small plain tip.
2. Let the cookies cool for about 10 minutes.  After that, you can decorate them with the royal icing, piping on snowflakes, hearts, or whatever ideas you can think of.  Let the cookies sit out until the icing hardens.  After that, you can place the cookies in an airtight container and keep them at room temperature for up to 3 days.  You can also freeze cookies for up to 2 months.