1 cup + 2 tablespoons all-purpose flour
1 1/4 cups dry yellow cake mix without pudding in it (I use Duncan Hines)
1/4 teaspoon salt
1/2 teaspoon baking soda
1 teaspoon baking powder
1/2 cup (1 stick) unsalted butter, at room temperature
1/4 cup brown sugar
1/2 cup granulated sugar
1 tablespoon vegetable oil
2 tablespoons light corn syrup
1 egg
1/2 teaspoon vanilla
1/2 cup mini chocolate chips
1/4 cup multicolored sprinkles, plus additional as needed (I get mine at the dollar store)
1/2 cup mini M&Ms (it does need to be the mini version here...I find that my beloved Target has the best price on these)
nonstick cooking spray
Preheat your oven to 350 degrees. Generously coat a 9x13 baking pan or a quarter sheet pan with nonstick cooking spray.
In a medium-sized bowl, whisk together the flour, dry cake mix, salt, baking soda, and baking powder. Set aside.
In a small dish, mix the 1/4 cup sprinkles and the M&Ms. Set aside 1/4 cup.
In a large bowl, beat the butter for about 30 seconds. Add both sugars and beat a minute or two until fluffy. Add the oil (measure it first), then the corn syrup (which will slip right out of your tablespoon if you measured the oil first), then the egg and vanilla to the butter-sugar mixture and beat until everyone seems to be getting along.
Dump the combined dry ingredients on top of the butter mixture and beat just until all the dry is in the party. Stir in the mini chocolate chips and the sprinkle/M&M mixture (minus your reserved 1/4 cup) by hand. This might take some muscle.
Plop large spoonfuls of your dough around your prepared pan, then spread evenly. Sprinkle on your reserved "confetti" mixture and press lightly with a spoon or your offset spatula.
If your bar cookies do not look carnival-y enough for you, dress them up with some extra sprinkles.
Bake at 350 degrees for 15-20 minutes or just until puffed in the center and starting to crack. The old toothpick test doesn't really work here: if you bake these until one tests clean, you will have less of a bar cookie and more of a very colorful concrete slab. Gooey is good in this case.
Cool on a wire rack for at least a couple minutes. Eat while warm if possible. If you need to store them, cool completely first, then cover the pan tightly with a lid or plastic wrap and foil.