Preheat the oven to 350°F (175°C). Grease the bottom only of a 16-cup metal tube pan or angel food cake pan with shortening.

In a bowl, whisk together the cake flour, baking powder, salt, and 1 cup (200 grams) of the granulated sugar.

Make a well in the center of the dry ingredients and add the oil, orange zest and juice, water, egg yolks, and vanilla extract. Beat on medium speed until completely smooth, about 1-2 minutes.

In a separate bowl, beat the egg whites and cream of tartar on medium speed until soft peaks form. Gradually add the remaining 1/2 cup (100 grams) of granulated sugar while increasing the speed to medium-high, and beat until stiff peaks form.

Gently fold one-third of the egg white mixture into the batter, then fold in the remaining egg white mixture until just combined.

Pour the batter into the prepared pan and smooth the top with a spatula.

Bake the cake for 40-45 minutes, or until the top springs back when lightly pressed and a toothpick inserted into the center comes out clean.

Invert the pan onto a wire rack or bottle to cool completely, about 2 hours.

Invert the cake onto a serving platter and run a sharp knife around the edge of the pan to loosen the sides. Gently remove the cake from the pan.

Sift powdered sugar evenly over the top of the cake.

Arrange orange slices on one side of the cake and serve.