Sift together flour, cocoa powder, baking soda, and salt. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat butter, 2 cups sugar, and eggs on medium speed until light and fluffy, about 2 minutes. Add vanilla, and mix to combine. Gradually add dry ingredients, and combine with mixer on low speed. Cover bowl with plastic wrap, and chill until dough is firm, about 1 hour.
Preheat the oven to 350 degrees. Line baking sheets with Silpat baking mats. Roll dough into 1-inch balls. Dip top of each ball into sugar. Place on prepared baking sheets about 1 1/2 inches apart. Bake until set, about 8 minutes. Cool on baking sheets for 5 minutes before transferring to a wire rack to cool completely.