Pre-heat the oven to 350ºF. If using a convection oven, set it to 325ºF.

Grease a 12-cup bundt cake pan with butter. You will need about 1 to 2 tablespoons to grease the pan. Make sure the butter is softened so it coats the pan evenly. Sprinkle flour all over the pan and shake it around to distribute evenly. If using a bundt pan with intricate details, make sure all the nooks and crannies are well coated so the cake doesn’t stick.

Make the orange sugar. Zest the oranges and mix the zest with the sugar, rubbing it between your fingers. This helps release the natural oils from the orange peel and makes the cake extra fragrant. Set aside.

In a medium bowl, whisk together the flour, cornstarch, salt, baking powder, and baking soda. Set aside.

In a large bowl, beat the butter with an electric mixer on medium speed for 2 minutes.

Add the orange sugar and beat again on medium-high speed until fluffy and lighter in color, about 2 minutes.

Add the eggs one at a time, mixing well after each addition.

Add the sour cream and orange juice and mix on low speed until combined.

Add half of the dry ingredients and mix on low speed until just incorporated.

Add the milk and mix briefly.

Add the remaining dry ingredients and mix until combined. Finish mixing with a spatula if necessary to avoid over mixing the batter.

Separate about ⅓ of the batter into a medium bowl. Add the cocoa powder and 1½ tablespoons milk to this portion and mix until smooth. If the batter is too light, or doesn’t have enough chocolate, add one more tbsp of cocoa and another ½ tbsp of milk to it. Avoid over mixing.

Spoon some of the plain orange batter into the prepared bundt pan, then spoon some of the chocolate batter on top. Continue alternating the batters, layering them throughout the pan until all the batter has been used.

Use a spatula or butter knife to gently swirl the batters together a few times to create a marbled effect. Don’t over-swirl, or the marble pattern will disappear.

Bake the cake in the pre-heated oven for about 55 to 65 minutes. Baking time will vary depending on your oven.

If the cake is browning too much on top before the center is fully baked, cover loosely with aluminum foil.

The cake is done when the top springs back lightly when touched and a toothpick inserted into the cake comes out clean.

While the cake bakes, make the orange syrup.

Place the sugar, orange juice, and orange peel in a small saucepan. Bring the mixture to a boil over medium heat.

As soon as the sugar dissolves completely, turn off the heat and set the syrup aside.

When the cake comes out of the oven, let it cool in the pan for about 5 minutes. Prickle the top of the cake with a fork.

Remove the orange peel from the syrup and slowly pour the syrup all over the warm cake.

Let the cake sit in the pan for another 15 minutes so it can absorb the syrup.

Then carefully flip the cake onto a cooling rack or serving plate and let it cool completely.

Place the heavy cream and orange peel in a small saucepan and heat over medium-low heat until hot and steaming, but not boiling.

Remove from the heat.

Strain out the orange peel and pour the hot cream over the chopped milk chocolate.

Let it sit for 1 minute, then whisk until smooth and glossy.

Allow the ganache to cool and thicken for about 30 minutes, until it becomes slightly firmer but still pourable.

Pour or drizzle the ganache over the cooled cake. Sprinkle with orange zest if desired.