1. Preheat oven to 350ºF. Lightly coat a 9" x 13" x 2" pan with nonstick cooking spray and line the bottom with parchment paper. In medium bowl, combine yolks, lemon extract, lemon juice, and lemon zest. Set aside.
2. Sift flour, sugar, baking powder, and salt into large mixing bowl. Add butter and milk; mix on low speed until batter is moist. Increase speed to medium and beat 1 minute. Add reserved yolk mixture in three batches, beating until well-combined.
3. Pour batter into prepared pan. Bake about 1 hour and 30 minutes, until cake is golden-brown and toothpick inserted in middle of cake comes out clean. Cool on wire rack.