Cake
3 cups all-purpose flour
1/2 teaspoon baking powder
1/4 teaspoon baking soda
1/2 teaspoon salt
1 cup (2 sticks) unsalted butter, at room temperature
3 cups granulated sugar
6 large eggs, at room temperature
1 teaspoon vanilla extract
Grated zest of two oranges
1 cup sour cream, at room temperature

Syrup
Grated zest of 1 orange
1 1/2 cups fresh orange juice
1 cup granulated sugar
3 tablespoons butter

To make the cake: Position a rack in the center of the oven and preheat to 325 degrees F. Butter and flour a 10-inch fluted tube or Bundt) pan, and tap out the excess flour.

Sift the flour, baking powder, baking soda and salt together; set aside.

Beat the butter and sugar in a large bowl with a hand-held electric mixer on high speed until light and fluffy, about 3 minutes. Beat in the eggs, one at a time, then the vanilla extract and zest. On low speed, add the flour in 3 additions, alternating with 2 additions of sour cream, beginning and ending with the flour. Beat until smooth, scraping down the sides of the bowl often with a rubber spatula. Spread evenly in the pan. Bake until a wooden skewer inserted in the center of the cake comes out clean, about 11/4 hours.

To make the syrup: Bring the orange zest, juice and sugar to a boil over medium-high heat and cook until reduced to a syrupy consistency, about 15 minutes.

Remove from heat and whisk in butter, 1 tablespoon at a time. Transfer the cake to a wire rack and cool for 10 minutes. Drizzle syrup over pound cake when you serve.