In a medium bowl, 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, sugar and eggs on medium speed until light and fluffy, about 2 minutes. Add vanilla and mix to combine. Gradually add dry ingredients, combine on low speed. Cover bowl with plastic wrap, chill until firm, about 1 hour.
Heat oven to 350 degrees F.
Line baking sheets with silpats (French nonstick baking mats) or parchment paper. 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. Transfer to a rack to cool. Cool on baking sheets for 5 minutes before transferring to a wire rack to cool. Repeat with remaining dough.
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