Preheat your oven to 350°F (175°C) and generously grease a 10-cup Bundt pan with butter or baking spray, making sure to coat all the fluted details to prevent sticking.
In the bowl of a stand mixer fitted with the paddle attachment, cream together 2 cups granulated sugar and 1 cup softened butter on medium-high speed for 3 to 4 minutes, until the mixture is light, fluffy, and pale yellow in color.
Beat in 4 room-temperature eggs one at a time, followed by 1 tablespoon of orange zest, mixing well after each addition until just combined and scraping down the sides of the bowl as needed.
In a separate medium bowl, whisk together 3 cups flour, 2 teaspoons baking powder, 1/2 teaspoon baking soda, and 1 teaspoon salt to evenly distribute the leavening agents.
With the mixer on low speed, alternately add the dry ingredients and 1 cup buttermilk to the butter mixture in three additions, beginning and ending with the dry ingredients, and mixing just until no large streaks of flour remain; finally, mix in 1/3 cup orange juice and 1 teaspoon vanilla extract until the batter is smooth.
Pour the thick batter into the prepared pan, smooth the top with a spatula, and bake for 45 to 55 minutes, or until a wooden skewer inserted into the center comes out clean with just a few moist crumbs attached.
Let the cake cool in the pan on a wire rack for 15 minutes, then carefully invert it onto the rack to cool completely for at least 1 hour before glazing to ensure the cake sets properly.
For the glaze, whisk together 1/2 cup confectioners' sugar and 2 tablespoons orange juice in a small bowl until smooth, then drizzle it over the completely cooled cake, allowing it to drip down the sides.