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1
To make the crust: Preheat oven to 325 F. Line an 8-inch square baking pan with aluminum foil, leaving an overhang on opposite sides to lift the bars out after they're baked. Spray the pan/foil with nonstick cooking spray.
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2
In a large bowl whisk the oats, brown sugar, flour, baking powder, baking soda, and salt together. Add the melted butter and use a rubber spatula to stir until all of the dry ingredients are moistened. Set aside about 3/4 cup of the crust mixture for the topping. Dump the remaining crust mixture into the prepared baking pan and press it into an even layer.
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3
Bake for 8 minutes, or until light golden brown. Transfer the pan to a wire rack and cool the crust completely, about 1 hour. Maintain oven temperature.
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4
To make the filling: In a medium bowl, whisk together the flour, sugar, espresso powder and salt. Add the chocolate and butter to a medium microwave-safe bowl. Heat in 30 second intervals at 50% power (stirring in between each interval) until the chocolate and butter are melted and smooth. Let cool slightly. Add the egg to the chocolate mixture and whisk to combine. Use a rubber spatula to fold in the flour mixture just until combined (the mixture will be thick).
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5
Spread the filling over the cooled crust. Smooth the top, then sprinkle the reserved oat topping over the filling. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs attached.
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6
Transfer the pan to a wire rack and let the bars cool completely in the pan, about 2 hours. Use the foil overhang to lift the bars out of the pan and cut into squares.
