Combine flour, cocoa, baking soda and salt in a small bowl.
Cream butter and sugar until light and fluffy. Beat in eggs and vanilla.
Slowly blend the dry mix into butter mixture.
Make small (teaspoonfuls) balls of the cookie dough. Press your thumb in the middle of the ball (like a thumbprint cookie). Put a small dollop of peanut butter in the indent. Press a little extra dough on top of the cookie to cover the peanut butter (its okay if a little is still showing). Place onto ungreased cookie sheet, 2 inches apart.