Lightly grease an 8x8-in. baking pan with butter. Set a sheet of parchment paper onto the bottom of the pan. Set aside.

Using a hand mixer or stand mixer fitted with a paddle attachment, beat butter, flour and powdered sugar until shortbread dough forms. Note: Dough should be easy to press together with fingers when ready.

Transfer dough to prepared pan, pressing it down and spreading out evenly. Freeze or refrigerate for about 10-15 minutes to chill.

Preheat oven to 350 degrees F. Remove crust from refrigerator and bake for 5-10 minutes or until it's slightly golden brown. Remove and cool.

In bowl, combine sugar, lemon juice, lemon zest, eggs, flour and arrowroot starch. Whisk until combined and smooth.

Pour filling into the slightly cooled crust. Return to oven and bake for 25-30 minutes, or until the top is almost set and center jiggles a little.

Remove from oven and allow lemon bars to cool completely. Cover and refrigerate to chill and set for at least 1 hour or overnight, if possible.

When ready to serve, dust with powdered sugar and cut into squares using a well-sharpened knife.