Preheat oven to 350°F. Generously grease and flour four 5 3/4" x 3 1/4" x 2" mini loaf pans and lay a strip of parchment paper along the bottom and up the two long sides of each pan, pressing the paper into the corners of the pan.
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 an electric 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 and bake for 30 to 35 minutes 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 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.
For glaze, whisk together powdered sugar and 1 1/2 tablespoons orange juice in a small bowl. Drizzle over cakes and allow to set and harden before wrapping loaves or storing them in an airtight container.
If you don't have buttermilk on hand, make your own by pouring 2 generous teaspoons of lemon juice or white vinegar into a measuring cup, and then adding milk up to the 3/4 cup line. Allow to sit at room temperature for about 10 minutes or until it begins to thicken and curdle.
Two large oranges gave me enough juice and zest for this recipe.
If making this recipe in two 8 1/2 x 4 1/2 x 2 1/2-inch loaf pans or one Bundt pan, start checking for doneness at 45 minutes. Cake is done when a toothpick inserted in center comes out clean (after probably no longer than an hour).