Preheat oven to 350 degrees. Line two large baking sheets with parchment paper or regular foil.
Thoroughly mix or sift the flour, cocoa, baking soda and salt. Set aside.
In a large mixing bowl, beat butter with sugar on medium speed until creamy.
Add eggs, beating at low speed until combined. Add vanilla.
Add flour mixture to creamed mixture and stir until combined. Stir in chips.
Drop tablespoon size mounds of dough onto baking sheets, spacing 2 1/2 inches apart. Alternatively, you can scoop the dough with a medium size cookie scoop.
Bake one sheet at a time on center rack for 12 minutes or until cookies appear set. Let cool for a few minutes on the baking sheet, then transfer to a wire rack.
When cookies are completely cool, melt some semisweet chocolate or chips using your favorite method. Drizzle melted chocolate over the cooled cookies, then sprinkle Reese’s Snack mix over the melted chocolate drizzle, poking pieces into the drizzle so that they stick.
Transfer cookies to the refrigerator and chill just long enough to set the chocolate drizze.