Preheat the oven to 350°F. Spray the bottoms and sides of two (8 inch) round cake pans with baking spray with flour or line the pans with foil.
In a large bowl, beat the cake mix, butter, 2 T. lemon zest and the egg with an electric mixer on low speed until crumbly. Beat on medium speed until dough forms. Press half of the dough in each pan.
Bake for 18 to 22 minutes or until the edges are light golden brown. Cool for 10 minutes.
In a small bowl, mix the glaze ingredients until smooth. Spoon the glaze over the warm shortbread; spread to the edges of the pans. Cool completely, about 45 minutes. Garnish with lemon zest, if desired. Cut each shortbread into 12 wedges.