Preheat the oven to 350℉. Line a 9x9-inch square baking pan with parchment paper or spray with nonstick cooking spray. Set aside.
In a small mixing bowl whisk together the flour, oats, baking soda, and salt. Set aside.
2 cups all-purpose flour, 1½ cups whole old-fashioned oats, 1 teaspoon baking soda, ¼ teaspoon salt
In a larger mixing bowl using an electric hand mixer, or use a stand mixer with the paddle attachment, beat the butter, peanut butter, granulated sugar, and light brown sugar on medium speed until light & fluffy. *Beat for at least 2 minute for the best fluffy texture and light color. 1/2 cup (1 stick) butter, 1/2 cup creamy peanut butter, 1/2 cup granulated sugar, 1/2 cup packed light brown sugar
Scrape down the sides of the bowl with a rubber spatula and beat in the eggs and vanilla extract just until mixed in.
2 large eggs, 2 teaspoons vanilla extract
Stir in the M&M's, chocolate chips, and peanut butter chips just until mixed in.* I recommend setting aside about 1/4 cup m&m's and some of the chocolate chips and peanut butter chips to decorate the tops of the bars with in the next step. 1 cup Ghoul's Mix Halloween M&M's, 1/2 cup chocolate chips, 1/2 cup peanut butter chips
Press the cookie dough into the prepared pan and smooth the top with a spoon or spatula. If you reserved some of the candy and chocolate chips, gently press those into the top of the bars.
Bake for 20-25 minutes or until the edges have set and a toothpick inserted into the center comes out clean.
Remove the pan from the oven. While the bars are still warm, immediately press the candy eyeballs into the bars. *Allow the bars to cool completely, inside the pan, before cutting them. About 30 minutes to 1 hour. candy eyeballs