In a large bowl, beat sugar and softened butter until light and fluffy.
Beat in eggs and vanilla.
In a separate bowl, combine flour, cocoa, baking soda, and salt. Add to butter mixture; beating until well combined.
Stir in peanut butter chips until distributed evenly into the cookie dough.
Drop by teaspoonfuls onto ungreased cookie sheet. Bake 9 to 10 minutes. Do not over-bake. The cookies will not look like they are done but they will puff up a little when baking and then they'll drop when they are cooling. They are soft with a little crunch around the edges.
Cool on the baking sheet for 10 minutes and then move them to a cooling rack. Store in an airtight container.