Beat ½ cup of softened unsalted butter, ½ cup of granulated sugar, and ½ cup of packed brown sugar until creamy. Beat in 1 room-temperature egg and 1 teaspoon of vanilla extract.
Whisk 1 cup of all-purpose flour, 10 tablespoons of natural unsweetened cocoa powder, 1 teaspoon of baking soda, and ⅛ teaspoon of salt. Mix into the butter mixture until combined, then fold in 1 cup of semisweet chocolate chips.
Cover the sticky dough tightly and refrigerate for at least 3 hours.
Preheat the oven to 350°F / 175°C and line baking sheets with parchment paper. Put 3 tablespoons of granulated sugar and 1 cup of icing sugar in separate bowls.
Shape the dough into 1½-tablespoon balls. Roll each lightly in the granulated sugar, then generously in the icing sugar, and space them 3in / 7.5cm apart on the baking sheets.
Bake for 11-12 minutes, until the edges are set and the centers still look soft. Cool on the baking sheets for 5 minutes, then move to a wire rack.
