Preheat oven to 350° F. Line a baking sheet with a silicone baking mat, or if not using, leave baking sheet ungreased.
In a medium bowl, sift gluten free flour blend, guar gum, salt, baking soda, and cocoa powder.
In another bowl (you can use your stand mixer if desired) combine room temperature butter, brown sugar and white sugar until well-blended. Add eggs, one at a time, mixing after each. Stir in vanilla.
Stir in gluten free flour mixture until well-combined.
Add the Reese's pieces and stir gently, being careful not to break the candies.
Taking about a golf ball size of dough, shape into a ball and place on a baking sheet, leaving about 2 inches in between each cookie.
Bake for 9-11 minutes at 350° F. Remove from oven and allow to cool for 3-4 minutes before transferring to a cooling rack. Repeat with remaining cookie dough. Store in an airtight container.