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