Preheat the oven to 350°F (175°C or 155°C for a fan oven).
Grease two 8½ x 4½ x 2½ loaf pans with a little butter.
Sift the flour, baking powder, baking soda and salt into a large bowl. Set aside.
Gently mix together ¼ cup of the orange juice, the buttermilk or pouring yogurt and vanilla in a pitcher or another bowl. Set aside.
Beat the butter and 1¾ cups of the sugar in an electric mixer until light and fluffy.
Beat in the beaten eggs and the orange zest.
Add the flour mixture and orange juice mixture alternately to the butter, sugar and egg mixture in the electric mixer. I do this in about 2 additions each, beginning and ending with the flour.
Carefully pour the batter into your two prepared pans, dividing the mixture evenly and gently smoothing the tops.
Bake for 45 - 60 minutes until the tops of the cakes are beginning to turn golden brown, the cakes spring back when touched lightly, and a skewer inserted in the centre comes out clean. Remove the cakes from the oven and cool in the pans for ten minutes.
Meanwhile, gently mix the remaining ¼ cup of sugar with the remaining ½ cup of orange juice in a small saucepan.
Heat the mixture gently, stirring constantly with a wooden spoon, until the sugar is completely dissolved.
Place a layer of wax or greaseproof paper on the table or counter top, with a layer of paper towels over it.
Remove the cakes from their baking pans to a wire cooling rack. Place the rack on top of the paper and paper towels.
Poke a few holes in the cakes with a skewer, and carefully spoon the syrup over top.
Let the cakes cool completely before glazing.
To make the glaze, put the icing sugar in a bowl and gradually add one to two teaspoons orange juice until a fairly thick, smooth and pourable consistency is reached.
Drizzle the glaze over the cakes.