For lemon curd, in a heavy saucepan, cook and stir the sugar, lemon juice and butter until smooth. Stir a small amount of hot mixture into egg; return all to pan. Bring to a gentle boil, stirring constantly; cook 2 minutes longer or until thickened. Stir in lemon zest. Cool for 10 minutes. Cover and chill for 1-1/2 hours or until thickened. , In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each. Add vanilla and lemon zest. Combine the flour, baking powder, baking soda and salt; add to creamed mixture alternately with buttermilk. , Fill paper-lined muffin cups three-fourths full. Bake at 350° for 20-25 minutes or until a toothpick comes out clean. Cool for 10 minutes; remove from pan to a wire rack to cool completely. , Cut a small hole in the corner of a pastry or plastic bag; insert a small round pastry tip. Fill bag with lemon curd. Insert tip 1 in. into center of each cupcake; fill with curd just until tops of cupcakes begin to crack. , In a small bowl, combine the butter, vanilla, salt, confectioners' sugar and enough milk to achieve frosting consistency. Frost cupcakes. Store in the refrigerator. Garnish with flowers or lemon zest if desired.