Position an oven rack in the center of the oven; preheat oven to 350°.
Lightly butter and flour a 9-inch fluted or plain tube pan and tap out the excess flour.
Cake-in a microwave oven on Medium (50%) power, in a microwave-safe bowl, heat the chocolates together for 30-second intervals, stirring after each interval, until melted, about 2 minutes; let cool until tepid.
In a bowl, whisk together the flour, baking soda, baking powder, and salt.
In a large bowl, beat the butter and sugar with an electric mixer set on high speed until light and fluffy, about 3 minutes.
One at a time, heat in the eggs, scraping down the sides of the bowl after each addition.
With the mixer on low speed, gradually add the flour mixture, and then the sour cream and vanilla.
Transfer half of the batter to a medium bowl.
Add tepid chocolate mixture and chocolate chips and stir well.
Add the orange zest to the plain batter.
Spoon the batters alternately into the prepared pan.
Using a table knife, swirl them together to create a marbled effect.
Bake until a long bamboo skewer inserted in the center of the cake comes out clean, about 50 minutes.
Let cool in pan on wire rack for 10 minutes.
Meanwhile, make the syrup: in a small saucepan, bring the orange juice and sugar to a boil, over medium heat, stirring often to dissolve the sugar.
Strain into a glass measuring cup.
Invert the cake onto the cooling rack and unmold.
Place the rack on a plate; slowly drizzle the orange syrup over the cake.
Using a dessert spoon, retrieve the syrup on the plate and drizzle it back over the cake.