Preheat oven to 350°F (176°C). Line a baking sheet with a silicone baking mat or parchment paper.
Combine flour, cocoa, baking soda and salt in a medium sized bowl. Set aside.
Cream butter and sugars (minus the 3 tablespoons of sugar for rolling) together until light and fluffy, about 2-3 minutes.
Mix in egg and vanilla extract.
Add the dry ingredients and mix until well incorporated and thick.
Roll one tablespoon sized balls of cookie dough. (NOTE: If you make larger balls of cookie dough, the cookies will spread more.)
Put the 3 additional tablespoons of sugar into a small bowl and roll each ball of cookie dough in it, coating the ball fully, then place the cookie dough balls on the lined cookie sheet.
Bake the cookies for 6-7 minutes.
Remove the cookies from the oven and press an unwrapped hershey kiss into the top center of each cookie. Allow cookies to cool for 2-3 minutes, then remove to cooling rack to cool completely.
