Add dry ingredients to a quart-sized mason jar one at a time, packing the ingredients as you go. Start with flour, salt, and baking powder, then brown sugar, sugar, and cocoa powder. Put Reese's Pieces on top.
When ready to make cookies, preheat the oven to 350ยบ. Scoop out the Reese's Pieces and set aside. Empty dry ingredients into a bowl. Add the wet ingredients and mix until combined. The dough will be soft. Fold in Reese's Pieces.
Form dough into 24 equal balls. Arrange on the cookie sheet and bake for 7-9 minutes. Allow to cool and enjoy!