Tip:This cake keeps extremely well, covered tightly at room temperature, for up to six days. It can also be frozen for up to two months.
Position an oven rack on the middle rung and heat the oven to 350°F. Butter a 9-inch bundt pan. With a hand mixer or a stand mixer fitted with the paddle attachment, beat the butter, sugar, and orange zest until light and fluffy, about 3 minutes. One at a time, add the egg and egg yolks (remember to reserve the whites), beating well and scraping down the sides of the bowl between each addition. Beat in the pumpkin and vanilla.
Sift together the flour, baking powder, and salt; stir in the cornmeal. Add this mixture to the pumpkin batter in three stages, stirring gently but thoroughly with a rubber spatula after each addition; don’t overwork the batter.
In a clean bowl with a very clean whisk or hand mixer, whip the reserved egg whites until they hold soft peaks. Gently fold the whites into the batter with the spatula until you no longer see streaks of white. Scrape the batter into the prepared pan; smooth the surface. Bake until the top of the cake is springy when lightly touched, the sides are beginning to pull away from the pan, and a wooden skewer inserted into the center of the cake comes out clean, 40 to 50 minutes. Let cool in the pan for 10 minutes and then invert the cake onto a wire rack to cool completely. Dust the cake with sifted confectioners’ sugar, if you like.
In a small saucepan over low heat, combine the orange juice and sugar, stirring until the sugar has dissolved. Increase the heat to medium high. Boil without stirring for 2 minutes. (If you make the syrup ahead, be sure to warm it gently before serving.)
Serve each slice of cake drizzled with syrup and, if you like, accompanied by a scoop of vanilla yogurt or vanilla ice cream.