Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.
Sift 1 1/2 cup flour with baking powder in a bowl. Cream margarine with 1/2 cup brown sugar in a separate large bowl, beating mixture until smooth. Stir flour mixture into creamed butter mixture and stir in 1/4 cup finely chopped pecans. Press the crust into the bottom of the prepared baking pan.
Bake crust in the preheated oven until hot, about 10 minutes.
Whisk eggs until foamy in a bowl; stir in dark corn syrup, 1/4 cup brown sugar, 3 tablespoons flour, salt, and vanilla extract. Whisk until the mixture is smoothly combined; pour over prebaked crust. Sprinkle with 3/4 cup pecans and return to oven.
Return to oven and bake until the topping is set, 25 to 30 minutes. Cool in the pan before cutting into bars.