Heat oven to 350°F (325°F for dark or nonstick pans). Spray bottoms and sides of two 8-inch round cake pans with baking spray with flour, or line with foil.
In large bowl, beat cake mix, butter, 2 tablespoons lemon peel and the egg with electric mixer on low speed until crumbly. Beat on medium speed until dough forms. Press half of dough in each pan.
Bake 17 to 23 minutes or until edges are light golden brown. Cool 10 minutes.
In small bowl, mix glaze ingredients until smooth. Spoon glaze over warm shortbread; spread to edges of pans. Cool completely, about 50 minutes. Garnish with lemon peel. Cut each shortbread into 12 wedges. Store loosely covered.