Preheat the oven to 160° C fan.
Melt the butter & set aside. Let it cool.
Line the baking tin crosswise w/ baking parchment. If necessary add some oil to the bottom & the sides to allow the baking parchment to stick appropriately.
Grate the orange zest. Squeeze the orange juice and add some more orange juice for getting 100 ml. Add the same amount of orange liqueur.
Whisk the eggs & the sugar w/ an egg whip.
Add the molten butter & whisk until well incorporated. It's rather liquid.
Mix the flour, the baking powder & the salt. Add the liquid mess & whisk until incorporated.
Add half of the the juice mix & whisk...
Fold in the orange zest w/ a spoon.
Pour the mess into the prepared baking tin & bake for about 35 min.
Make the famous test w/ the wooden pin... If necessary add another 5 - 10 min to the oven session.
Mix the rest of the juice mix w/ sugar and heat it up. The sugar has to dissipate.
With a wooden pin make small holes all over surface of the cake - just after the oven session. With a pastry brush distribute the hot juice mix all over the cake until all liquid is soaked up.
Let the cake rest on the cooling rack until it is at room temperature before removing the baking tin & the baking parchment.