Bake Sale Trifecta Bars

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For the brownie layer:

1 stick unsalted butter, at room temperature, plus more for the pan

1 cup granulated sugar

1/3 cup packed light brown sugar

 2 large eggs

2 teaspoons pure vanilla extract

3/4 cup all-purpose flour

1/2 cup unsweetened cocoa powder

1/4 teaspoon salt

For the cookie layer:

1 stick unsalted butter, at room temperature

3/4 cup packed light brown sugar

 2 tablespoons granulated sugar

1 large egg

1 teaspoon pure vanilla extract

1 1/2 cups all-purpose flour

1/2 teaspoon baking soda

1/2 teaspoon salt

1 cup chocolate chips

For the cereal-treat layer:

3 tablespoons unsalted butter, plus more for your hands

1 10-ounce bag mini marshmallows

3 cups crispy rice cereal