How to Make It
In a medium bowl, whisk first 7 ingredients. In a large bowl, using an electric mixer on medium speed, cream butter and brown sugar until fluffy, about 2 minutes. Add molasses and egg yolk; beat until smooth. Add dry ingredients on low speed, until just combined. Pat dough into a disk, wrap in plastic wrap and chill at least 2 hours (up to 2 days).
Place racks in upper and lower thirds of oven and preheat to 350°F. Line 2 baking sheets with parchment. Lightly flour work surface and rolling pin. Roll dough to 1/4-inch thickness. Cut out cookies and arrange on baking sheets, 2 inches apart. Gather dough scraps and cut out more cookies to fill sheets.
Bake for 12 to 15 minutes, rotating sheets halfway through. Let cookies cool on sheets for 10 minutes, then transfer to wire racks to cook completely. Repeat to bake remaining dough.