Preheat the oven to 350ºF (180°C).
Butter or spray 24 mini bundt pans. For other pan sizes, see Notes below. With an electric mixer beat soft 9 ounces unsalted butter and 1 cup sour cream until smooth in a large bowl, about 1 minute.
Add 1 tablespoon orange zest and mix.
Add 3 cups sugar gradually and beat until light and fluffy, about 3 minutes more.
Add 6 eggs, one at a time, incorporating well each one before adding the next.
Mix in 1 teaspoon vanilla extract and ¼ cup orange juice.
Sift 3 ¼ cups all-purpose flour with ¼ teaspoon baking soda and ¼ teaspoon salt and add it to the butter mixture in 2 additions, beating at low speed only until well mixed. Don’t overbeat. You can sift them before or have the ingredients measured and sift them directly over the butter mixture (my choice).
Pour into the pans, dividing evenly. Use small spoons for this as the molds are small.
Bake for about 20 minutes, or until golden and a cake tester or toothpick comes out clean. The time will depend on the pans you use.
Let cool for 10 minutes in the pan on a wire rack, and then unmold.
Let cool completely on the cooling rack before glazing.