In a large bowl, combine flour, sugar, baking powder and salt. In a small bowl, lightly beat eggs; add milk, butter and lemon zest. Pour into dry ingredients and stir just until moistened. Spoon half of the batter into 12 greased or paper-lined muffin cups. Make a well in the center of each; add jam. Spoon remaining batter over jam. Bake at 400° for 20-25 minutes or until golden.