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1
In a heavy saucepan, bring the sugar and orange juice to a boil over moderately high heat, stirring.
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2
Add three-quarters of the rhubarb and the orange zest and simmer, stirring, until the rhubarb is softened and slightly thickened, about 5 minutes.
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3
Add the remaining rhubarb and cook until just tender, about 2 minutes.
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4
Let cool.
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5
Preheat the oven to 375.
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6
In a bowl, sift together the flour, baking powder and salt.
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7
In a large bowl, beat the egg yolks with 1 1/2 cups of the sugar and the orange zest on high speed until light and fluffy, about 3 minutes.
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8
Gradually beat in the oil.
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9
Working in batches, alternately fold in the flour mixture and the orange juice, beginning and ending with the flour mixture.
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10
In a large bowl, beat the egg whites with the cream of tartar until very soft peaks form.
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11
Add the remaining 1/4 cup of sugar and beat until firm, glossy peaks form.
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12
Fold one-third of the whites into the cake batter until incorporated, then fold in the remaining whites just until blended.
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13
Scrape the batter into a 10-inch tube pan with a removeable bottom and lightly smooth the top.
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14
Tap the pan once to release any air pockets.
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15
Bake the cake in the bottom third of the oven for about 30 minutes, or until the top springs back when lightly pressed.
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16
Invert the cake pan onto a jar and let cool in the pan.
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17
Run a thin metal knife between the cake and the sides of the pan to loosen.
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18
Invert the cake onto a cake plate and serve with the rhubarb jam.
