In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
Measure buttermilk, orange juice, and vanilla into a measuring cup. Set aside.
In a large bowl, use a mixer to beat the butter until creamy.
Blend in sugar and beat for a few minutes until light and fluffy.
Mix in eggs, one at a time, and orange zest, and beat until well incorporated.
With mixer on low, slowly pour in 1/3 of the flour mixture.
Mix in half of the buttermilk mixture.
Blend in another 1/3 of the flour mixture and remaining buttermilk mixture.
Add remaining flour mixture and mix until just blended.
Divide batter between prepared pans, filling about 3/4 full, and bake for 30 to 35 minutes (45 minutes if using a large loaf pan) or until a toothpick inserted in center comes out clean.
While loaves bake, prepare orange simple syrup.
Stir together 1/2 cup orange juice and 1/2 cup brown sugar in a small pot. Bring to a boil over medium heat while occasionally stirring, then reduce heat and simmer for a few minutes. Remove pot from heat and set aside to cool.
When cakes are done, cool for 10 minutes before turning out onto wire rack. Set rack over a sheet pan or piece of foil, and using a brush or a spoon, soak each cake with simple syrup. Allow to cool completely.
Store in an airtight container or wrap tightly.
Source: Foodista.com – The Cooking Encyclopedia Everyone Can Edit