Adjust an oven rack to the middle position. Heat oven to 375. Spread 1/2 cup sugar in a shallow dish for coating. Set aside. Whisk the flour, baking soda, spices, pepper and salt together in a large bowl. Set aside.
Beat butter, brown sugar and remaining 1/3 cup sugar together in a large bowl using an elecctric mixer on medium speed until light and fluffy, 3-6 inutes. Beat in egg yolk and vanilla until combined, about 30 seconds. Beat in the molasses until incorporated, about 30 seconds, scraping down the side of the bowl and bveater as needed.
Reduce mixer speed to low. Slowly mix in the flour mixture until combined, about 30 seconds. (Dough will be soft.) Give the dough a final stir with a rubhger spatula to make sure it is combinewd.
Using wet hands, roll 2 T. dough at a time into balls, then roll in the sugar to coat. Lay on two parchment-lined baking sheets, spaced about 2 inches apart. Bake, one sheet at a tim, until edges set and tops are cracked but centers are still soft and underdone. (Peek thru cracks to check centers.) It's about 10-12 minutes. Rotate the baking sheet halfway thru baking.
Let cookies cool on sheet for 10 minutes. You can serve them at this point or transfer to a wire rack, cooling completely before putting on the rum glaze.
To make glaze, just mix the confectioners sugar and rum together until smooth. Drizzle it over the cooled cookies. Let set for 10-15 minutes before serving.
NOTE: if you want to make these w/o the rum glaze, you could also use a variation that has orange essence in it. Just add 1 t. grated orange zest to the dough when you add the molasses. Process 2/3 cup sugar with 2 t. grated orange zest in a food processor until fragrance, about 10 seconds. Use that for rolling instead of the plain white sugar.