In a large bowl, whisk together the flour, baking powder, 3/4 cup of the sugar, and salt; set aside. In a medium bowl beat together the oil, egg yolks, orange zest, orange juice and vanilla. Add the liquid to the flour mixture and beat until smooth; set aside.

In the bowl of a stand mixer (or in a medium bowl using a handheld mixer), beat the egg whites until foamy. Add the cream of tartar and beat until the stiff peak stage. Gradually beat in the remaining cup of sugar. Continue beating until the egg whites are very stiff and glossy.

Fold one-third of the egg whites into the flour mixture. Gently fold in the remaining egg whites until just combined. Pour the batter into an ungreased 10-inch angel-food cake pan or tube pan. Bake in a 350º F oven until the centre of the cake springs back when lightly touched, about 55 to 60 minutes. Remove the cake from the oven and invert the pan onto a large heatproof funnel or bottle. Cool the cake completely upside-down in the pan before removing. Serve with fresh fruit and whipped cream.