Place a rack in the center of the oven and preheat the oven to 350 degrees F. Grease a 9x13-inch pan with cooking spray or line it with parchment paper, leaving an overhang on the sides to lift the finished bars out. Set aside.
Make the crust: in the bowl of a stand mixer fitted with the paddle attachment or a large mixing bowl, beat the butter and granulated sugar on medium-high speed, until light and fluffy, about 2 minutes. Scrape down the bowl. Beat in the eggs and the vanilla, just until incorporated.
In a separate large bowl, whisk together the flour, baking powder, and salt. With the mixer on low speed, slowly add the dry ingredients to the butter mixture, stopping as soon as the flour disappears.
Transfer the dough to the prepared pan. Press it evenly into the pan, building it up on the sides all the way around by about 1-inch. The dough will be very sticky, so if it clings to your hands, use a sheet of plastic wrap to press it down and avoid sticking. The crust will seem fairly thick, but go with it. Bake the crust for 15 minutes, until it is set but not yet browned. If it puffs up unevenly in the center, use the tines of a fork to prick it lightly on the surface. Set aside to cool.
While the crust cools, prepare the filling: in a large, heavy-bottomed saucepan, combine the butter, maple syrup, brown sugar, and lemon zest.
Cook over low heat, stirring the mixture with a wooden spoon, until the butter is melted. As soon as the butter melts, increase the heat to a boil. Boil for 3 minutes (set a timer!), then remove from the heat.
Stir in the heavy cream, vanilla, and chopped pecans. Avoid the temptation to touch or eat the pecan filling, as it is very very hot. Pour the filling carefully into the center of the crust, then with the back of a spatula or fork, spread it into an even layer (some of the crust may show at the edges; try to avoid the batter seeping between the crust and the pan). Sprinkle a generous pinch of flaky salt over the top. It will be quite thick and fill the pan almost all the way to the top.
Bake for 25 to 30 minutes, until the filling is set at the edges and when the pan is jiggled, the center has some movement but does not seem excessively liquidy. Place on a cooling rack and let cool completely to room temperature, then cover the bars with plastic and place in the refrigerator to chill for at least 6 hours, preferably overnight. With a knife, loosen the bars from the sides of the pan, then lift them onto a cutting board (see blog post above for tips). Slice into bars of the desired size. Enjoy at room temperature or slightly warmed in the microwave (my favorite!).
TO STORE: You do not need to refrigerate pecan pie bars, if you will be enjoying them within a day or two. These keep well at room temperature for up to 2 days and in the refrigerator for up to five days.
TO FREEZE: Lay fully cooled bars onto a parchment-lined baking sheet, and place in the freezer until frozen solid. Then, place the frozen bars into a freezer-safe ziptop bag or airtight container and store in the freezer for up to 3 months.
TO SERVE AFTER FREEZING: If the bars are frozen, let them thaw overnight in the refrigerator. Let them come to room temperature prior to serving or warm them up a little in the microwave. If you do not have a microwave, you can try warming them in a low oven. Tent the pan with foil to keep the top from over browning.