How to Make It
Preheat oven to 350°.
To prepare cake, weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours, xanthan gum, and next 3 ingredients (through salt) in a medium bowl; stir with a whisk.
Place granulated sugar and 1/4 cup butter in a large bowl; beat with a mixer at medium speed until well blended. Add eggs, 1 at a time, beating well after each addition. Beat in canola oil, lemon rind, and lemon juice. Add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture.
Pour batter into 2 (8-inch) round cake pans coated with cooking spray; sharply tap pans once on counter to remove air bubbles.
Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes in pans on a wire rack; remove from pans. Cool completely on wire rack.
To prepare frosting, place cream cheese and 3 tablespoons butter in a medium bowl; beat with a mixer at medium speed until light and fluffy. Add powdered sugar, 1 cup at a time, beating until well blended. Beat in 1 teaspoon lemon rind, 2 teaspoons lemon juice, and vanilla until smooth.
Place 1 cake layer on a plate; spread with 1/2 cup frosting. Top with second cake layer. Spread remaining frosting over top and sides of cake. Store cake loosely covered in refrigerator.