Preheat oven to 350°. In a large bowl, cream butter and 2/3 cup sugar until light and fluffy, 5-7 minutes. Add eggs, 1 at a time, beating well after each addition. Beat in yogurt, lemon juice, zest and extract.
In another bowl, whisk flour, baking powder and baking soda. Add to creamed mixture; stir just until moistened. Fill greased or paper-lined muffin cups three-fourths full. Bake 17-19 minutes or until a toothpick inserted in center comes out clean. Remove from the oven. Increase oven setting to 400°.
Meanwhile, in a small bowl, beat egg whites on medium speed until soft peaks form. Gradually add remaining sugar, 1 tablespoon at a time, beating on high after each addition until sugar is dissolved. Continue beating until stiff glossy peaks form.
Spread or pipe meringue onto muffins. Bake 6-8 minutes longer or until meringue is golden brown.
Cool 5 minutes before removing from pan to a wire rack. Serve warm. Refrigerate leftovers.