Preheat oven to 350°F. Spray 8 x 8 in. square pan with vegetable pan spray.

In medium bowl, combine flour, baking powder, baking soda and salt; mix well. In large bowl, beat butter and sugar with electric mixer until light and fluffy. Add eggs, lemon juice and vanilla; mix until smooth. Add flour mixture to butter mixture alternately with milk, beating well after each addition. Stir in lemon zest. Place batter in prepared pan.

Bake 30-33 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes on cooling rack. Remove from pan; cool completely.

For icing, beat shortening and butter in large bowl with electric mixer until light and fluffy. Add lemon juice and zest; mix well. Gradually add sugar, one cup at a time, beating well on medium speed. If icing is dry, add additional lemon juice or milk and beat at medium speed until light and fluffy. Color as desired with icing color. Ice cake and if desired, garnish with fresh lemon zest curls and mint.