Preheat the oven to 350 degrees F. Butter a 9-inch fluted tube pan and set it aside. Cream the butter, sugar, and orange zest together until light and fluffy. Add the whole egg and the two egg yolks, one at a time, beating well and scraping down the sides of the bowl between each addition. Beat in the pumpkin puree.

Sift together the flours, baking powder, and salt. Add to the pumpkin batter in three or four stages, blending gently but thoroughly after each. Stir in the final addition of dry ingredients by hand if you have been using a mixer, so as not to overwork the batter. In a clean small bowl, whip the egg whites until they hold soft peaks. Fold into the batter, then scrape the batter into the prepared pan and smooth the surface. Bake in the center of the oven for 50 to 60 minutes, or until the top of the cake is springy when lightly touched, the sides are beginning to pull away from the sides of the pan and a wooden skewer inserted in the center of the cake comes out clean. Cool in the pan for 10 minutes, then invert onto a wire rack and cool completely. This cake keeps very well for several days, well wrapped, and it freezes beautifully for up to 2 months.

For the syrup, combine the orange juice and sugar in a small saucepan over low heat and stir until the sugar has dissolved. Increase the heat, bringing the syrup to a boil. Boil without stirring for 2 minutes, then use immediately, or allow to cool and refrigerate for up to 3 days (reheat gently).

To serve, place one slice of the pumpkin orange cake on each plate. Spoon a few tablespoons of the warm syrup over each piece, and accompany with a scoop of thick vanilla yogurt (for breakfast) or ice cream (for dessert).