Preheat oven to 350 degrees F. Line an 8x8" baking pan with aluminum foil or parchment paper, lightly sprayed with cooking spray. Set aside.
Combine crust ingredients (melted butter, flour, powdered sugar and cornstarch) in a mixing bowl, then press into bottom of prepared pan. Keep crust in an even layer, as much as possible. Bake 10 minutes, then set aside.
While crust is baking, combine filling ingredients (eggs, sugar, zest, juice, flour, buttermilk, and baking powder), and whisk to combine. Try not to whisk too vigorously, as you don't want to incorporate a lot of air into the filling.
Pour over baked crust and bake (still at 350 degrees F), for 22-25 minutes, until filling has set and edges are lightly golden brown.
Cool completely, cover with foil, then refrigerate.
To slice bars, I find it easiest to lift up on the foiling lining in the pan, to remove the bars from the pan. Slice into 12 bars. Before serving, dust bars with powdered sugar and garnish with a lemon slice if desired.