Sift together flour, cocoa powder, baking soda, and salt into a bowl.
In a bowl of an electric mixer fitted with the paddle attachment, beat butter and sugar on medium speed until light and fluffy, about 2 minutes. Add eggs and vanilla, and beat to combine. Reduce speed to low, and gradually add flour mixture; beat to combine. Form dough into a flattened disk, wrap with plastic wrap, and chill until firm, about 1 hour.
Preheat oven to 350F. Shape dough into 1 1/4 inch balls. Roll each ball in sugar. Place on baking sheets lined with parchment paper, about 1 1/2 inches apart. Bake until set, 10 to 12 minutes, rotating halfway through. After you remove from oven let cool on sheet for 5 minutes, Transfer to a rack to cool completely. Cookies can be stored between layers of parchment paper in an airtight container at room temerature for up to 1 week.