Preheat the oven to 430°F (220°C). To prepare the mold, use a pastry brush and well-softened unsalted butter to grease a bundt pan (a decorative fluted tube pan). Make sure to grease the sides of the pan with its intricate patterns (little nooks and crannies) and the central funnel.

Sprinkle the baking pan with granulated sugar by swirling it to coat. Shake the excess sugar. Refrigerate the mold until ready to use.

To make the cake batter, sift plain flour, cornstarch, potato starch, and baking powder in a large bowl and mix with a hand whisk.

Place egg yolks and 1/2 cup + 1/2 tablespoon (120 g) of granulated sugar in the bowl of a stand mixer or a mixer bowl. Using a stand or an electric mixer, beat until the mixture whitens and leaves a ribbon-like trail once lifted.

Add the sifted dry ingredients to the egg yolk mixture and mix on low speed. Add the scraped seeds of vanilla beans and orange zest and mix again.

In a separate bowl, whisk room-temperature egg whites with a pinch of salt and cream of tartar until soft peaks. Gradually add the remaining sugar (60 g) and whisk to get a firm glossy meringue.  Gently fold the beaten egg whites into the main preparation with a rubber spatula. 

Pour the cake batter into the prepared tin up to two-thirds. Tap it on the table a few times to release large air bubbles and fill the decorative pattern of the mold. 

Bring the pan into the oven and immediately lower the temperature to 320°F (160°C). Bake for 20 to 25 minutes until well-risen and golden brown. Check for doneness with the blade of a knife or a cake tester: if it comes out dry, the cake is ready. 

Let the cake rest in the pan for a few minutes and unmold it on a wire rack to cool. To serve, sprinkle the cooled Savoy cake with powdered (icing) sugar.