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1
For the crust and topping: Adjust an oven rack to the middle position and heat the oven to 325 degrees. Line an 8 inch square baking pan with a foil sling and grease the foil.
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2
Whisk the oats, brown sugar, flour, baking powder, baking soda, and salt together in a large bowl.
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3
Stir in the melted butter until combined.
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4
Reserve 3/4 cup of the oat mixture for the topping.
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5
Sprinkle the remaining oat mixture into the prepared pan and press into an even layer with the bottom of a measuring cup.
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6
Bake the crust until light golden brown, about 8 minutes.
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7
Let the crust cool completely on a wire rack, about 1 hour.
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8
For the filling: Whisk the flour, sugar, instant espresso, and salt together in a medium bowl.
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9
Melt the chocolate chips and butter together in a microwave, stirring often, 1 to 3 minutes.
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10
Transfer the chocolate mixture to a large bowl and let cool slightly.
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11
Whisk in the egg until combined.
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12
Stir in the flour mixture until just incorporated.
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13
Adjust an oven rack to the middle position and heat the oven to 325 degrees.
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14
Spread the filling evenly over the cooled crust and smooth the top.
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15
Sprinkle with the reserved oat topping.
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16
Bake the bars until a toothpick inserted into the center comes out with a few moist crumbs attached and the filling begins to pull away from the sides of the pan, 25 to 30 minutes, rotating the pan halfway through baking.
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17
Let the bars cool completely in the pan, set on a wire rack, about 2 hours.
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18
Remove the bars from the pan using the foil, cut into squares, and serve.
