I tested these with gfJules and Gluten-free Mama's almond flour blend. Both worked well. If using gfJules, or another blend with xanthan already in it, then omit the additional xanthan called for.
You can make this using your stand mixer, a hand mixer, or just mix by hand in a large bowl.
In a large bowl, combine gluten-free oats, brown sugar, gluten-free flour, xanthan (if needed), baking soda, and salt.
Stir in melted butter and pure vanilla extract until you have a moist, crumbly mixture.
Measure out 1 cup of this crumble mixture and set aside to use as the topping later.
Spray a 9x13 baking pan with cooking spray. Press oat mixture evenly into bottom of pan. I like to use some plastic wrap or a plastic glove sprayed with cooking oil to help me do this easily.
Bake at 3750 for 12 minutes. Remove from oven and let rest on cooling rack or hot pads for 5 minutes.
In a small bowl, whisk together the sweetened condensed milk and creamy peanut butter until well mixed. Carefully spread over the baked crust.
Sprinkle with M&M's and chocolate chips.
Drop small pieces of the reserved 1 cup of oat mixture all over the top of chocolate. My dough pieces were roughly dime-sized and smaller. You'll still be able to see much of the layers underneath.
Bake 24-25 minutes at 3750 or until some parts are turning golden brown. Remove from oven to cooling rack. Let cool completely before cutting. I cut mine in a 4x6 grid, yielding 24 bars. But these are very rich, so you can definitely cut even smaller if you want to feed more people!