Place a rack in the center of the oven and preheat to 350 F degrees. Place two sheets of parchment paper into the baking pan, perpendicular to each other. Brush with vegetable oil or melted butter.
Using a stand mixer with a paddle attachment mix on low all the dry ingredients, about 5 seconds.
On low speed add the butter a piece at a time. Slowly mix until a dough forms and begins to pull away from the side. About 2 minutes.
Press dough into the bottom of prepared pan and about ½" up the sides. Chill at least 30 minutes.
Prick bottom of crust with a fork and bake until light golden brown, about 15 minutes.
Allow the crust to cool for 15 to 20 minutes before adding the filling. Turn down the oven to 300 F degrees.
Strawberry Lemon Custard
Purée strawberries in a blender until smooth. Strain through a fine-mesh sieve into a medium bowl until you get ⅓ cup of puree. Reserve any remaining liquid for another use.
Whisk eggs, granulated sugar, and lemon juice and zest in a medium bowl. Pour in strawberry puree and whisk until smooth.
Whisk in the flour until just combined.
Pour filling into crust and bake until filling is set and slightly golden at the edges, about 25 to 30 minutes.
Allow the bars to cool completely in the pan on a wire rack. Then refrigerate until chilled through. About two hours.
Using the parchment paper pull the entire pan of bars out at once. Dust lightly with powdered sugar and cut into squares.
Xanthan gum- If your flour blend does not contain xanthan gum add ¼ teaspoon to the shortbread crust. none is needed for the filling.
Storing Instructions: The bars can be refrigerated in an airtight container in the refrigerator for 3 days. Bars should be in a single layer and not stacked.
Freezing Instructions: Bars can be frozen for up to 2 months. Cut the cooled bars into squares, then place onto a baking sheet and freeze for 1 hour. Separate each bar with parchment and place them into a large freezer bag. Thaw the lemon bars in the refrigerator overnight. Dust with powdered sugar before serving (i.e. don't do this before freezing).