1 1/3 cups unsalted butter
1/3 cup packed brown sugar
1/3 cup white sugar
3/4 cup packed light brown sugar
1 teaspoon baking soda
1 1/2 teaspoons lemon zest
1 pinch salt
3 eggs
1 teaspoon vanilla extract
Preheat the oven to 375 degrees F (190 degrees C). Grease and flour a 9x13 inch pan. Sift together the flour, brown sugar, and white sugar. Set aside.
In a large bowl, cream together the butter and brown sugar until a smooth mixture is formed. Mix together the white sugar, baking soda and lemon zest. Beat in the eggs, one at a time then stir in the vanilla. Beat in the flour mixture. Fold in the flour mixture. Pour batter into prepared pan.
Bake in the preheated oven for 30 to 35 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool slightly before frosting. For an even spread, frost with whipped cream while pan is baking. Cover and refrigerate until ready to frost. Frost with frosting from the outside of the pan. Chill at least 2 hours before use.