2 1/2 cups sifted all-purpose flour
Preheat oven to 350 degrees. Grease and flour and 8 cup-kugelhopf or bundt pan.
Sift together the flour, baking powder, baking soda, and salt, and set aside.
Beat the butter in an electric mixer until soft. Add the sugar and beat to mix. Add the eggs one at a time, beating after each addition. On low speed, add the sifted dry ingredients in three additions, alternating with buttermilk, scraping bowl as necessary.
Remove from the mixer and stir in the zest and nuts.
Pour into the prepared pan, smooth top, and place in the hot oven. Bake for 55-60 minutes, until top springs back when pressed lightly.
Remove from the oven and set on a rack.
Immediately prepare the glaze.
Place the juices and sugar in a saucepan over moderate heat and stir with a wooden spoon until the sugar is dissolved and the mixture comes to a boil. Remove from heat, add the rum, and stir.
Pierce the top of the cake with a cake tester. Spoon the hot glaze over the hot cake (still in the pan), spooning a little at a time. When you notice glaze oozing around the edge of the cake pan, use a metal spatula or knife to ease the edge of the cake away from the pan, allowing the glaze to run down the sides. Continue this until all the glaze is absorbed. It will be absorbed, believe me.
Let the cake stand for 10 to 15 minute, until the bottom of the pan is cool enough to touch. It would take a lot longer. Then cover the cake with a plate, hold the plate tightly in place against the cake pan, and flip over the cake and pan. Remove the cake pan from the cake. Let stand for at least two hours until cool and cover with plastic wrap.
Better if made a day ahead and freezes beautifully.