Preheat oven to 350ºF. Prepare a 13x9-inch pan with parchment paper (let edges overhang as handles) and spray with nonstick spray.
In a large bowl, cream 1 cup butter and 1/3 cup brown sugar together. Add 1 egg and 1 tsp. of vanilla, beating until thoroughly combined. Add flour, flax seed, and salt and mix until just combined. Fold in pecans. Scoop dough into prepared pan, pressing mixture evenly into bottom of pan using moistened fingertips or a spoon or spatula. Bake crust in a 350°F oven for 15 minutes or until the edges are golden brown. Remove from oven and set aside.
In a large bowl, beat corn syrup, 1/2 cup brown sugar, 2 eggs, melted butter, 1 tsp. of vanilla, and salt with electric mixer until light and fluffy, about two minutes. Fold in pecans. Pour topping over baked crust and return oven to bake 30 to 35 minutes longer or until praline layer is set. Remove from oven and immediately sprinkle with chocolate chips. Let melt and soften 1-2 minutes, then spread chocolate evenly over the top with a knife or offset spatula. Set pan aside to cool to room temperature before serving (or once cool to the touch, place in refrigerator to cool more quickly.)
Once cooled, carefully lift cooled bars from the pan using the parchment paper as handles. Cut bars with a sharp knife.
Leftovers can be kept in an airtight container for up to four days.