These are delicious! Recipe courtesy of The America's Test Kitchen Family Baking Book. You can use old-fashioned rolled oats, but the bars will be more chewy. Make sure you cool the crust before you add the filling.
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.
Whisk the oats, brown sugar, flour, baking powder, baking soda, and salt together in a large bowl.
Stir in the melted butter until combined.
Reserve 3/4 cup of the oat mixture for the topping.
Sprinkle the remaining oat mixture into the prepared pan and press into an even layer with the bottom of a measuring cup.
Bake the crust until light golden brown, about 8 minutes.
Let the crust cool completely on a wire rack, about 1 hour.
For the filling: Whisk the flour, sugar, instant espresso, and salt together in a medium bowl.
Melt the chocolate chips and butter together in a microwave, stirring often, 1 to 3 minutes.
Transfer the chocolate mixture to a large bowl and let cool slightly.
Whisk in the egg until combined.
Stir in the flour mixture until just incorporated.
Adjust an oven rack to the middle position and heat the oven to 325 degrees.
Spread the filling evenly over the cooled crust and smooth the top.
Sprinkle with the reserved oat topping.
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.
Let the bars cool completely in the pan, set on a wire rack, about 2 hours.
Remove the bars from the pan using the foil, cut into squares, and serve.