Crust and Topping:
- 3/4 cup all-purpose flour
- 1 cup quick-cooking oatmeal
- 1 cup packed light brown sugar
- 1/4 teaspoon baking powder
- 1/4 teaspoon baking soda
- 1/8 teaspoon salt
- 8 tablespoons (1 stick) unsalted butter, melted and cooled slightly
- 1/2 cup coarsely chopped salted, roasted cashews
- 1/2 cup all-purpose flour
- 1/2 cup packed light brown sugar
- 1 tablespoon instant espresso powder
- 1/2 teaspoon ground cinnamon
- 1/4 teaspoon chipotle chili powder
- 1/4 teaspoon salt
- 12 ounces premium semisweet chocolate, chopped
- 4 tablespoons unsalted butter, chopped
- 3 large eggs
For the crust: Adjust oven rack to middle position and heat oven to 325 degrees F. Line a 9-inch square metal baking pan with foil, leaving an overhang for easy removal. Spray foil with nonstick cooking spray.
Whisk the flour, oatmeal, brown sugar, baking powder, baking soda and salt in a large bowl. Stir in melted butter until combined. Set 1 1/4 cups oatmeal mixture aside. Press remaining oatmeal mixture firmly into prepared pan. Bake until light golden brown, about 8 minutes. Cool completely. Stir cashews into the reserved oatmeal mixture.
For the filling: Whisk the flour, brown sugar, instant espresso powder, cinnamon, chili powder and salt in a medium bowl. Put the chocolate and butter in large microwavable bowl; microwave on medium power until chocolate is softened, stopping every 30 seconds to stir. Stir until mixture is melted and smooth. Whisk in eggs, then stir in flour mixture. Pour filling over cooled crust and sprinkle with reserved oatmeal mixture.
Bake until toothpick inserted into center comes out with few crumbs attached, 35 to 40 minutes. Cool to room temperature, at least 1 hour. Remove using foil overhang and cut into squares.