8 Tbsp unsalted butter, cold, cut into ½-inch pieces
3 egg whites
½ tsp vanilla extract
1½ cups powdered sugar, divided
¾ cup fresh lemon juice
2 tsp lemon zest
Preheat oven to 350 degrees.
To make crust, mix whole-wheat flour and brown sugar using a food processor until fully mixed. Toss butter into flour mixture and pulse to combine using short bursts of power until butter is cut into tiny pea-sized pieces and dough appears lumpy.
Sprinkle crust mixture onto an ungreased 13 X 9 X 2-inch baking pan as evenly as possible; press down with a spatula to create a packed surface.
Place crust in middle of oven and bake until golden, about 20 minutes.
While crust is baking, to make lemon topping, beat egg and egg whites in a medium bowl with an electric mixer until well blended.
Add vanilla extract and ¾ cup of powdered sugar; mix well.
Add lemon juice and remaining powdered sugar; mix until powdered sugar is completely dissolved and then add zest.
As soon as crust is finished, remove from oven and reduce oven temperature to 300 degrees.