- 1 stick unsalted butter softened
- 1 tsp lemon zest grated
- 1 cup sugar
- 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.