Grease a cookie sheet; set aside. In a large mixing bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add sugar, cocoa powder, baking soda, and salt. Beat until combined, scraping sides of bowl occasionally. Beat in egg and vanilla until combined. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour. Cover and chill dough for 1 hour or until easy to handle.
Shape dough into 1-inch balls. Roll balls in sprinkles. Place balls 2 inches apart on prepared cookie sheet. Press your thumb into the center of each ball.
Bake in a 375 degrees oven for 7 to 8 minutes or until edges are firm. Remove from oven; immediately press an unwrapped chocolate kiss into the center of each cookie. Transfer cookies to a wire rack and let cool.