Add both butters from crust and filling ingredient list to a medium saucepan over medium heat. Cook, stirring occasionally, until the butter turns golden brown, smells nutty, and the milk solids darken. Pour the brown butter into a heat-safe bowl or large measuring cup set on a food scale and divide it evenly — half for the crust and half for the filling. Let cool slightly, about 10 minutes.

Preheat the oven to 350°F (177°C). Line an 8×8-inch baking pan with parchment paper.

In a mixing bowl, whisk together the flour, brown sugar, salt, and vanilla. Add half of the brown butter and stir until the mixture becomes crumbly and sandy.

Note: The texture will feel slightly oily and pasty. If it looks overly wet, mix in 1 tablespoon of additional flour.

Press the crust evenly into the prepared pan and bake for 10 minutes.

While the crust is baking, wipe the mixing bowl clean.

Add the remaining brown butter, brown sugar, and maple syrup to the bowl and whisk until combined.

Add the eggs, vanilla, salt, and flour, whisking until smooth.

Fold in the chopped pecans.

Remove the crust from the oven and immediately pour the pecan filling over the warm crust.

Return the pan to the oven and bake for 22–28 minutes, or until the center is set but still has a slight jiggle when the pan is gently shaken.

Let the bars cool in the pan for 10 minutes.

Lift them out using the parchment paper and transfer to a cooling rack. Cool for at least 2 hours before slicing for the cleanest cuts.