In a small mixing bowl, whisk together flour, salt, and baking soda.

In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat together the butter and sugars until very light and fluffy, about 4-5 minutes.

Add the eggs and vanilla and beat for 2 more minutes on medium-high speed, until the eggs are well incorporated and the mixture is fluffy.

Add the dry ingredients as well as the M&Ms, Reese's Pieces, and chopped Hershey bar and mix on low speed until the flour has been totally mixed in and no longer. Don't overmix!

Lightly grease a 9x13 inch baking pan and using lightly greased hands, press the cookie dough in an even layer into the baking pan. Refrigerate for 20 minutes.

Preheat an oven to 350 degrees F. Bake the bars for 25-27 minutes, until golden brown and puffy. Cool for 15 minutes and then press the remaining chopped candy into the top of the bars. Top with sugar-candy eyes for extra Halloween vibes!

Cool to room temperature before slicing into bars.