- Preheat the oven to 350 degrees F. Grease a square baking pan. Beat sugar and butter together in a mixing bowl using an electric mixer until light and fluffy. Beat in eggs and vanilla extract. Sift flour and baking powder together in a separate bowl; add to creamed mixture. Pour in milk, lemon zest, and lemon juice and mix until you achieve a smooth batter. Spoon batter into the prepared pan. Bake in the preheated oven about 35 minutes. Cut into pieces and serve.