You can make this cake up to 2 days ahead and keep it chilled.
Preheat oven to 350°.
To prepare cake, coat a 9-inch square baking pan with cooking spray; line bottom of pan with wax paper. Coat wax paper with cooking spray.
Lightly spoon flour into dry measuring cups; level with a knife. Sift together flour, baking soda, cinnamon, and salt.
Place buttermilk and next 5 ingredients (buttermilk through egg) in a large bowl; beat with a mixer at medium speed until well blended. Add flour mixture to buttermilk mixture, stirring just until combined.
Spoon batter into prepared pan. Bake at 350° for 35 minutes or until wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack. Loosen cake from sides of pan with a narrow metal spatula; remove from pan. Peel off wax paper; cool completely on a wire rack.
To prepare frosting, place powdered sugar, lemon rind, and cream cheese in a bowl; beat with a mixer at medium speed until smooth. Spread the frosting over top of the cake. Cut into squares.