Preheat oven to 325 degrees. Grease and flour a 10" Bundt pan.
Sift flour, baking powder, baking soda and salt together; set aside.
Beat butter and sugar in large bowl with electric mixer until light and fluffy, about 3 minutes. Beat in eggs, 1 at a time, and then beat in vanilla and zest.
On low speed, add flour in 3 additions, alternating twice with sour cream (begin and end with flour).
Beat until smooth, scraping side of bowl with rubber spatula. Spread evenly in prepared pan.
Bake about 1-1/4 hours, or until a tester inserted in the center of the cake comes out clean. Cool on rack 10 minutes, then unmold.
Make syrup while cake is cooling:
In a small saucepan, bring zest, juice and sugar to a boil over medium-high heat. Cook 15 minutes until syrupy. Remove from heat and whisk in butter, 1 tablespoon at a time. Drizzle warm syrup over cake slices.