A little sanding sugar (coarse, super-sparkly sugar found in baking supply stores) or smoked salt are a fun finishing touch for these delicious cookies.
In a medium bowl, whisk both flours, the cocoa, baking soda, and salt. In a large bowl, using an electric mixer (or in a stand mixer fitted with the paddle attachment), beat the butter, brown sugar, granulated sugar, and vanilla on medium speed until smooth, about 2 minutes. Add the dry ingredients. Beat on low speed until just blended.
Divide the dough in half. Roll each half into a 1 1/2-inch-wide, 8-inch-long log. Wrap tightly in plastic and place on a baking sheet. Freeze for at least 2 hours or up to a week.
Arrange racks in the top and bottom thirds of the oven; preheat to 350°. Line 2 baking sheets with parchment paper. Using a sharp knife, cut the logs of dough into 1/2-inch-thick slices. Arrange half the slices, spaced 2 inches apart, on the baking sheets. Bake until just set but still soft, about 10 minutes. (Overbaking cocoa cookies will make them taste bitter.)
Transfer the baking sheets to wire racks. Let the cookies cool completely on the sheets. (Don’t be tempted to pull off the cookies while they’re still warm. These cookies taste better and are much sturdier when they’re completely cool.) Let the sheets cool, then line with fresh parchment. Slice and bake the remaining dough.
Place the chopped chocolate in a metal bowl. Set the bowl over a saucepan of simmering water. Stir the chocolate until melted and smooth. (Or microwave the chocolate in 20-second bursts, stirring often, until melted and smooth.) Let the chocolate cool slightly for about 5 minutes.
To decorate the cookies, place a wire rack in each of 2 rimmed baking sheets. Dip the cookies halfway into the melted chocolate and set on the racks to let any excess drip off. Just before the chocolate is set, sprinkle the cookies with smoked sea salt or sanding sugar (if using).