Preheat oven to 300º.
In a medium sized bowl, combine flours, cocoa, soda and salt with a wire whisk.
In an electric mixer, blend together the softened butter and sugars, making sure to scrape down the sides of the bowl.
Add eggs and vanilla. Beat at medium speed until smooth.
Add the flour mixture and blend at low speed. When almost combined, add Reese's Pieces and mix until just combined. Don't overmix!
Drop by tablespoon or use a cookie scoop to place cookies on an ungreased cookie sheet, 2 inches apart. Bake for 18-22 minutes. Transfer cookies to a cooling rack as soon as they're out of the oven.