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1
Put the oven rack in the middle position and preheat the oven to 350u00b0F. Brush 2 (9-inch) round cake pans with canola oil. Line the bottom of each pan with a round of parchment or wax paper, then oil the paper.
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2
Assemble the flour mix: put all ingredients into a sealable airtight container and shake until combined well. Whisk together the flour mix, salt, baking powder, and xanthan gum until combined well. Stir together the milk, canola oil, vanilla, and zest in another bowl.
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3
Beat together the sugar and eggs in a large bowl with an electric mixer at medium speed just until combined, about 1 minute. Reduce the speed to low and add the flour and milk mixtures alternately in batches, beginning and ending with the flour mixture and mixing until just combined.
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4
Divide the batter evenly between the cake pans, smoothing the tops, and bake until a wooden pick or skewer inserted in the center of each cake layer comes out clean, 35 to 40 minutes.
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5
Cool the cake layers in pans on racks 10 minutes. Run a thin knife around edge of one cake layer and invert rack over cake pan, then invert cake onto rack. Repeat with the second layer.
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6
Peel off the paper and cool layers completely.
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7
Whisk together the zest, lemon juice, sugar, yolks, a pinch of salt, and the guar gum in a 1-quart heavy saucepan. Add the butter and cook over moderately low heat, whisking constantly, until the curd is thick enough to hold the marks of a whisk and the first bubbles appear on the surface, about 5 minutes. Whisk in the extract.
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8
Immediately pour the hot curd into a bowl, then cover the surface with wax paper and chill until cold, about 30 minutes.
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9
Beat the butter with an electric mixer at high speed until light and fluffy, about 1 minute. Reduce the speed to low and add the confectioners' sugar, lemon juice, extract, and zest, then mix until creamy and smooth, about 2 minutes.
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10
Halve each cake layer horizontally using a long serrated knife. Spread the bottom half of each cake layer with half of the lemon curd, then top with remaining cake layers to form two sandwiched cakes.
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11
Put one sandwiched cake on a cake stand or platter and spread a heaping 1/2 cup frosting over top, then cover with remaining sandwiched cake. Frost the top and sides of cake with remaining frosting.
