In a large bowl, whisk together flour, cocoa powder, baking soda and salt. Set aside.
In a separate bowl, use an electric mixer to beat butter and ¾ cup sugar together until light and fluffy, about 2 minutes. Add egg, egg yolk and vanilla. Mix until well combined, scraping down the sides and bottom of the bowl as necessary. Add flour mixture to wet ingredients and mix just until combined.
Cover the bowl and refrigerate the dough for 2 hours or ideally overnight.
Remove the cookie dough from the refrigerator and let is sit at room temperature while the oven preheats to 350 F. Line a baking sheet with parchment paper and set aside.
Use a medium cookie scoop to scoop a heaping tablespoon of dough. Roll dough into a ball in your hands. Roll dough ball in granulated sugar, then confectioners’ sugar generously. Place on cookie sheet and repeat with remaining dough, spacing each dough ball 2 inches apart.
Bake cookies at 350° F for 10 to 12 minutes. The cookies will look puffy, but will flatten slightly as they cool. Allow cookies to cool on the pan for 5 minutes before transferring to a wire rack to cool completely.
Notes
Store cookies in an airtight container at room temperature for up to 5 days.Freeze cookie dough in plastic wrap or an airtight container for up to 3 months. Defrost in the refrigerator before scooping into balls and baking.