To toast the pecans: Preheat oven to 350°. Place pecans in a single layer In a 9” x 9” baking pan. Bake in a 350 degree oven for 5 - 10 minutes until golden brown and fragrant. Shake every few minutes and watch closely. Allow to cool.
Meanwhile, melt 4 tablespoons butter. Set aside and allow to cool for the filling.
For the crust: Line an 8-inch square pan with foil (two large sheets going opposite direction), then coat lightly with oil spray.
Process flour, 1/3 cup brown sugar, pecans, salt and baking powder in a food processor until it resembles coarse cornmeal, about 5 pulses. Sprinkle the 6 tablespoons chilled butter cubes over the top. Pulse until the mixture is pale yellow and resembles coarse oatmeal, about 8 pulses.
Place the crust mixture in the prepared pan. Press firmly into an even crust with your fingers. Bake until it begins to brown, 20 - 24 minutes.
For the filling: As soon as the crust finishes baking, stir together the 1/2 cup brown sugar, corn syrup, melted butter, bourbon/rum or rum extract, vanilla extract and salt just until combined. Whisk in the egg until incoporated, then pour over the hot crust. Sprinkle the 1 3/4 cups chopped pecans over the top.
Return to oven and bake until top is golden brown and top begins to crack, about 22 to 25 minutes.
Cool completely on a wire rack, about 1 hours, before removing from the pan using the foil as lifters. Cut as desired.