1 cup all-purpose flour spooned and leveled, plus more for pan
2 large eggs separated
1/2 tsp pure vanilla extract
1/4 tsp kosher salt
1 cup walnuts toasted and coarsely chopped
1/4 tsp cream of tartar
1 tbsp fresh lemon juice
Preheat the oven to 350 degrees. Butter an 8-by-8-inch baking dish.
In a large mixing bowl, cream the butter with ½ cup sugar until light and creamy. Add the egg yolks, one at a time. Add the lemon zest, salt, and vanilla and mix until combined. Slowly add the flour and ½ cup walnuts.
Evenly spread the mixture in the baking dish. Bake for 20 minutes. Remove from the oven but leave the oven on.
Beat the egg whites and cream of tartar until stiff. Gradually beat in the remaining ½ cup sugar. Fold in the lemon juice and the remaining ½ cup walnuts. Spread over the hot baked layer.
Bake for 25 minutes, until lightly browned. Cool completely before cutting.