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Put oven rack in middle position and preheat oven to 350°F. Brush cake pans with canola oil. Line bottom of each pan with a round of parchment or wax paper, then oil paper.
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Whisk together flour mix, salt, baking powder, and xanthan gum until combined well. Stir together milk, canola oil (1 cup), vanilla, and zest in another bowl.
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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 flour mixture and mixing until just combined.
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Divide batter evenly between cake pans, smoothing tops, and bake until a wooden pick or skewer inserted in center of each cake layer comes out clean, 35 to 40 minutes.
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Cool cake layers in pans on racks 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.