Line two baking sheets with parchment. In a medium bowl, sift together flour, ground ginger, cinnamon, cloves, nutmeg, and cocoa.
In the bowl of an electric mixer fitted with the paddle attachment, beat butter and grated ginger until lightened, about 4 minutes. Add brown sugar; beat until combined. Add molasses; beat until combined.
In a small bowl, dissolve baking soda in boiling water. Beat half of flour mixture into butter mixture. Beat in baking-soda mixture, then remaining half of flour mixture. Mix in chocolate; turn out onto a piece of plastic wrap. Pat dough out to about 1 inch thick; seal with wrap. Refrigerate until firm, 2 hours or overnight.
Heat oven to 325 degrees. Roll dough into 1 1/2- inch balls; place 2 inches apart on prepared baking sheets. Refrigerate 20 minutes. Roll in granulated sugar. Bake until the surfaces crack slightly, 10 to 12 minutes, rotating halfway through. Let cool on sheets 5 minutes, then transfer cookies to a wire rack and let cool completely. Cookies are best the day they are made but can be stored in an airtight container at room temperature up to 5 days.