Preheat oven to 350°F (175°C).

Grease a loaf pan.

Melt butter in a double boiler or microwave.

In a mixing bowl, combine melted butter and sugar.

Beat the butter and sugar mixture well until light and fluffy.

Add egg yolks and orange zest to the butter mixture.

Beat until well combined.

Sift flour and baking powder together in a separate bowl.

In another clean bowl, beat egg whites until stiff peaks form.

Gradually add the flour mixture to the butter mixture, alternating with the beaten egg whites.

Gently fold until just combined; do not overmix.

Pour the batter into the prepared loaf pan.

Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean.

While the cake is baking, prepare the glaze.

In a saucepan, combine sugar and water.

Bring the mixture to a boil over medium heat.

Reduce heat and simmer for 5-6 minutes, or until the sugar is dissolved and the mixture has slightly thickened.

Remove from heat and stir in orange juice and zest.

Once the cake is done baking, remove it from the oven.

Pierce the cake all over with a fork.

Slowly pour the hot syrup over the top of the cake, allowing it to soak in.

Return the cake to the oven for a few minutes to allow the glaze to set.

Remove the cake from the oven and let it cool completely in the pan before removing and slicing.