Put oven rack in middle of oven, and preheat oven to 350 degrees F. Line two 9-inch cake pans with parchment paper, and grease sides of pan & paper. Set aside.
Whisk together flour mixture, salt, baking powder & xanthan gum until combined. Stir together milk, canola oil, vanilla and zest in another bowl.
Beat together sugar and eggs in a large bowl with an electric mixer at medium speed just until combined, about 1 minute. Reduce speed to low and add flour and milk mixtures alternately in batches, beginning and ending with the flour mixture and mixing until just combined.
Divide batter evenly between cake pans, smoothing tops, and bake until a wooden pick or skewer inserted into center of each cake layer comes out clean, about 35-40 minutes.
Cool cakes in pans on racks for 10 minutes. Run a thin knife around edge of 1 cake layer and invert rack over cake pan, then invert cake onto rack. Repeat with second layer. Peel off paper and cool layers completely.
Cakes can be baked a day in advance, and wrapped tightly with plastic wrap once cooled completely.
Whisk together juice, zest, sugar, and eggs in a 2-quart heavy saucepan. Stir in butter and cook over moderately low heat, whisking frequently, until curd is thick enough to hold marks of whisk and first bubbles appears on surface, about 6 minutes.
Transfer lemon curd to a bowl and chill, its surface covered with plastic wrap, until cold, at least one hour.
Lemon Whipped Cream Frosting
In a medium bowl, whip whipping cream until soft peaks form.
Beat in confectioners sugar. Whip until stiff peaks form.
In a large bowl, place 2 cups of lemon curd. Take a scoop of the whipped cream, and stir it into the lemon curd (this lightens it). Next, fold the remaining whipped cream into the lemon curd.
Sprinkle 2 Tbsp of pudding powder over the top of the mixture. Using a whisk, stir it into the frosting. It will thicken up instantly, giving you a stable, light frosting.
Using a long serrated knife, halve each cake layer horizontally. Spread bottom half of each cake layer with 1 cup of lemon curd, then top with remaining cake layers to form two sandwiched cakes. Put on sandwiched cake on a cake stand or platter and spread a heaping 1/2 cup of frosting over top, then cover with remaining sandwiched cake. Frost top and sides of cake with remaining frosting. Cover and store cake in refrigerator. Remove from fridge 1/2 hour before serving.
Featherlight Flour Mix: 3 cups Rice flour (white or brown), 3 cups Tapioca starch, 3 cups Cornstarch, 3 Tbsp Potato flour (note: flour, not starch). Lemon Curd can be chilled up to 1 week. The Lemon Curd recipe makes approximately 4 cups of curd. You can use 1 cup to sandwich each cake layer (if necessary), and 2 cups to stir into the frosting. To cover the cake with plastic wrap, insert toothpicks into the cake to keep the plastic wrap from touching the cake.
Recipe by The Baking Beauties at http://www.thebakingbeauties.com/2010/03/gluten-free-layered-lemon-cake-with-lemon-whipped-cream-frosting.html