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Position a rack in the centre of the oven and preheat to 325ºF. Line 2 ungreased 8-inch round cake pans (see blog post for more information) with rounds of parchment paper cut to fit. The cake batter will crawl up the sides of the ungreased pan as it bakes, making the cake extra fluffy.
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In a large bowl, whisk together the oil, water, egg yolks, and vanilla. Then whisk in 1/2 cup (100 gr) sugar.
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Place a strainer over the bowl and sift the sweet rice, millet and oat flours with the baking powder and salt directly into the yolk mixture, adding back any bits left behind in the strainer. Whisk until very smooth.
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In the clean, dry bowl of a stand mixer fitted with the whip attachment, whip the egg whites with the cream of tartar on medium-high speed until soft peaks form. Slowly add the final 2 tbsp sugar and whip until the whites are glossy and just hold a firm peak when lifted from the bowl and turned upside down, 1–3 more minutes.
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Use a flexible silicone spatula to fold one-third of the whites into the batter, then gently fold in the remaining whites until just combined and no streaks remain.
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Immediately pour the batter into the prepared pans and quickly but gently use an offset spatula to spread the batter into a thin, even layer.
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Transfer the pans to the oven and bake until the cakes are golden, spring back to the touch, and a toothpick inserted near the centre comes out clean, 20-30 minutes. Remove to a rack and let cool completely in the pan.
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Run a knife around the edge between the cake and the sides of the pan. Loosen and invert the pan to release the cake.
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Cool and then wrap until needed. The cake can be made a day in advance and stored in an airtight container till needed. Alternatively, wrap well, place in a freezer bag and freeze for up to six months.