Beat butter, peanut butter, and sugar in a mixing bowl. Then add in egg, milk, and vanilla. Mix until light and fluffy.
In a separate bowl sift cocoa, flour, baking soda, and salt. Once sifted add flour mixture to sugar mixture and beat until just combined.
Shape dough into 1 inch balls. Set on a parchment-lined baking sheet.
Place on prepared cookie sheet about 2 inches apart. (If want them extra chewy, chill for 10 minutes in the freezer before baking.)
Bake for 8 to 10 minutes.
While baking unwrap Hershey’s Kisses. Once cookies are done baking, remove them from the oven and immediately press a chocolate Kiss into the center of each cookie. The cookie should crack around the edges.