Preheat oven to 350°F (180°C). Grease and flour a 6-cup bundt pan.
In a bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt. Set aside.
In a stand mixer, beat butter until creamy. Gradually add sugar, followed by orange and lemon zest. Beat until pale and fluffy, 4–5 minutes.
Add eggs one at a time, mixing well after each. Stir in vanilla and sour cream until combined.
On low speed, add flour mixture in three parts, alternating with orange juice, starting and ending with flour. Do not overmix.
Transfer batter to prepared pan, smoothing the top. Bake 25–30 minutes, until a toothpick inserted comes out clean.
For syrup (optional): Simmer orange juice and sugar until dissolved. Poke holes in warm cake with a skewer, brush one-third syrup on top, let cool 10 minutes, then invert cake onto a platter. Brush remaining syrup over cake.
For glaze (optional): Whisk cream cheese until smooth. Add powdered sugar, then cream until thick yet pourable. Drizzle over cooled cake.
Slice and serve, garnished with orange peel if desired.