In a large bowl, beat butter, brown sugar, sour cream, egg, and vanilla on medium speed of an electric mixer until smooth.
Beat in the flour, cocoa, baking soda, and salt on low speed until all the ingredients are mixed.
Shape the dough into 1 1/4-inch balls. Place 3-inches apart on ungreased baking sheet. Flatten into 2-inch circles with the bottom of a glass dipped in the granulated sugar.
For each cookie, break 2 pretzels in half. Place the pretzel into the dough to resemble legs. Decorate with the candy eyeballs and peanut butter candies.
Bake for 7 to 9 minutes or until the edges are set.
Cool on the cookie sheet for 2 minutes. Remove the cookies to a cooling rack and cool completely (about 20 minutes).