Sift together flour, cocoa powder, baking soda, and salt in a bowl.
In the bowl of an electric mixer fitted with the paddle attachment, beat sugar and butter on medium speed until mixture is 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, and chill until firm, about 1 hour.
Preheat oven to 350 degrees.
Shape dough into 1 ¼-inch balls.
Roll each ball in sanding sugar.
Place on baking sheets lined with parchment paper, about 1 ½ inches apart.
Bake until set, 10 to 12 minutes, rotating halfway through.
Transfer to a rack to cool for 5 minutes.
Transfer cookies from baking sheet to wire rack.
The cookies can be stored between layers of parchment in an airtight container at room temperature for up to 1 week.
