Preheat oven to 350°F (180°C). Line baking sheets with parchment paper.
In the bowl of a stand mixer, In the bowl of a stand mixer fitted with the paddle attachment, beat butter, granulated sugar, and brown sugar at medium speed until fluffy, about 3 minutes, stopping to scrape sides of bowl. Add egg and egg yolk, and beat for 2 minutes. Beat in vanilla.
In a medium bowl, whisk together flour, cocoa, salt, and baking soda. Add flour mixture to butter mixture, and beat at low speed until just combined. (Dough will be thick.) Using a 3-tablespoon spring-loaded scoop, scoop dough, and place 3 inches apart on prepared pans.
Bake, one pan at a time, for 13 minutes. Let cool on pan for 3 to 5 minutes. Remove from pan, and let cool completely on a wire rack.