Line an 8x8-inch pan with parchment paper. Pour oats and cereal into a large bowl; set aside.

In a medium saucepan combine butter, brown sugar, corn syrup, and peanut butter. Heat over medium heat until sugar is dissolved, stirring constantly. Bring to a boil and remove from heat. Add the dry ingredients to the pan and stir until combined.

Pour mixture into lined pan and use a rubber spatula or wooden spoon to even out the layer before pressing down. Immediately sprinkle the mini M&Ms evenly over the entire surface. Working quickly (mixture will begin to soften and melt M&Ms), use your hands to press mixture down gently to secure M&Ms into place. Then sprinkle the chocolate chips evenly over the granola bars, pressing down more firmly and quickly since chocolate chips will begin to melt once they hit the bars. Allow bars to cool completely*.

Remove oat mixture from pan and cut into 12 bars. Melt the peanut butter chips and shortening together in the microwave until smooth. Use a spoon to drizzle over the bars and place in the refrigerator until peanut butter drizzle has set, about 10 minutes. Wrap bars individually in plastic wrap, or store them in an airtight container at room temperature.