Preheat the oven to 350°. Lightly spray a 9X13-inch baking pan with cooking spray and set aside.I line my pan with parchment paper (so there is overhang on the sides) which makes it really easy to grab the bars out and cut them nicely.
In a small bowl add the flour, oats, baking soda, and salt. Stir with a wire whisk to break up any clumps. Set aside to use later.
In a large bowl, or in the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, peanut butter, brown sugar, and sugar until fluffy looking and light in color, 2-3 minutes.
Add in the vanilla, egg and egg yolk. Mix together.
Add the bowl of dry ingredients and mix, on low speed, just until combined.
Add the chopped candy bars, m&m's, and sprinkles. Stir together with a wooden spoon or spatula until combined.
Press the batter into the prepared pan and spread it out evenly. It's easier to do this if you dump piles of the dough all over the pan, rather than in one pile, and then use your fingers to spread it out.
Gently press additional m&m's, chopped candy bars, and sprinkles onto the top of the bars (optional).
Bake for 15-18 minutes. Be careful to not overbake these! Only the very edges should be a very light golden brown color, while the center is still pale in color. These will cook and set up more as they cool. I have never had to cook mine for longer than 15 mintues.
Let the bars cool in the pan for about 30 minutes before cutting. Store leftovers, covered, at room temeprature. These freeze well too once they are baked.