Preheat the oven to 350 degrees. Prepare a 13x9 inch baking pan by lining with foil, letting the flaps hang over the sides for easy removal after baking. Set aside.
In a food processor fitted with the dough blade, pulse together the flour and powdered sugar. Add the butter and pulse until a crumbly mixture forms. {You can also stand mixer or two knives or a pastry cutter to "cut in" the butter.}
Press the mixture into the prepared baking pan, patting it down with your hands {or the bottom of a measuring cup} to create an even base. Bake for about 20 minutes or until the edges just begin to turn golden.
Meanwhile, make the filling: In a large mixing bowl {or the bowl of a stand mixer}, beat the sweetened condensed milk and eggs until it turns a pale yellow color and looks thickened. Add vanilla bean paste, citrus zest, citrus juice, flour, baking powder and salt and beat until it's fully combined.
Pour the mixture into the crust. Bake for about 20-23 minutes or until the filling is set {it shouldn't wiggle} and the top is lightly golden.
Remove from oven and cool until you can handle the pan, then place in the refrigerator and chill for about 2 hours.
When ready to serve, cut into bars and sprinkle with powdered sugar to garnish.