This recipe is adapted from one by America's Test Kitchen. Being in Boston, they probably know a thing or two about how these cookies are supposed to be.
3 ¼cupsall-purpose flour (measure with a light hand or weigh)(420 grams)
¾teaspoonground ginger
⅛teaspoonground cloves
¼teaspoonnutmeg
½teaspoonallspice
8tablespoonsunsalted butter, softened(114 grams)
1cupgranulated sugar(200 grams)
Instructions
In a glass measure, stir together the molasses and baking soda and let sit for about 10 minutes. Molasses will expand in volume.
Meanwhile, combine the rum, water and salt. Stir to dissolve salt. If it doesn't all dissolve, don't worry. At this point you can also mix together the flour and all spices.
In a mixing bowl, beat the butter and sugar until well blended. It won't be super creamy due to the high amount of sugar. Scrape in the molasses/soda mixture and beat until mixed, then gradually add the rum mixture and the flour mixture alternately. The batter starts out very thick, so it will take some elbow grease. If you have a stand mixer with a paddle, this is a good use for it.
With a cookie scoop, scoop out about 18 or 24 portions of dough. At this point the mounds of dough will be pretty soft. Arrange them on two dinner plates, cover with plastic and chill overnight or at least for a few hours.
When ready to bake, shape the chilled rounds into neat balls. Pat them down ½ inch thick (about 3 inches across) on parchment lined baking sheets about 6 to a sheet and leaving plenty of room for spreading.
Bake one sheet at a time in a 375 degree oven for about 10 minutes (check at 8 if you made them smaller). Allow the cookies to cool for about 10 minute directly on the baking sheet.
When the cookies cool they'll be fairly firm, but as they stand they soften up. I stored mine in a zipper bag for several days and they seemed to get softer and softer. You can bend them almost into taco shell shape.