Set the oven rack in the middle position and heat the oven to 350 degrees. Line a 13×9” baking dish with a sheet of foil as wide as the pan and about 22 inches long, pushing it into the corners and up the sides at either end; there should be a couple of inches of over hang at both ends. Spray with nonstick cooking spray and set the pan aside.
In a food processor, blend the flour, brown sugar, baking powder, salt, and the pecans to combine, about 6 quick pulses.
Scatter the cold butter pieces over the flour mixture and pulse to combine, about 12 quick pulses. The mixture should resemble a coarse meal.
Pour the mixture into the prepared pan and pat into a tight, even layer. Make sure it is not too thin near the center or thick around the edges. Otherwise the filling mixture will pool in the center and you won’t have enough filling in the bars cut from the edges.
Bake until crust is golden brown about 35 minutes, rotating the pan halfway through the baking time.
To make the topping: In a large bowl, whisk the brown sugar, corn syrup, melted butter, vanilla, espresso powder, and salt to combine.
Add the eggs and whisk to combine.
Pour the mixture onto the hot crust and spread evenly. Scatter the pecans evenly over the filling, pressing them gently in to the filling and bake until filling is set and nuts are browned and toasted, about 25 minutes, rotating pan halfway through baking time.
Remove from oven, set the pan on a wire rack, and cool to room temperature at least 2 hours.
Use the ends of the foil as handles to lift the entire thing out of the pan and place it onto cutting board. Use a sharp chef’s knife to cut into 2×3” bars and serve.