Preheat the oven to 180 °C / 355 °F. Grease a loaf pan (approx. 20 x 11 x 7.5cm / 8 x 4.5 x 3 inches) with butter, then line with parchment paper.
Make the cake
In a small bowl combine all-purpose flour, a pinch of salt and baking powder. In a large bowl (or in a bowl of a stand mixer) whisk together butter (room temperature), sugar and zest of three bio oranges. Mix for about 5 minutes on medium speed, in a stand mixer or using an electric mixer. Gradually add the eggs, one by one, waiting until the previous egg has been completely incorporated before adding the next one. Add the dry ingredients (flour, salt, baking powder) and mix well to get a nice, smooth mixture, then add the freshly squeezed orange juice and orange liqueur (optional) and incorporated well.
Transfer the orange cake mixture to your prepared loaf pan. Spread evenly and tap the baking dish against your counter to get rid of any air bubbles. Place in a preheated oven and bake for 45 - 50 minutes or until the skewers come out clean.TipIf you insert it into the center of the cake, it should come out clean, with no streaks of batter.
Drizzle with orange juice
Transfer the baked orange cake from the oven and let it sit for about 10 minutes in the loaf pan. Using a skewer make little holes over the whole upper surface. Drizzle with four tablespoons of freshly squeezed orange juice, then transfer from the loaf pan to a wire rack to cool completely.
Make the glaze. Add icing sugar to a bowl. Strain the freshly squeezed orange juice through a sieve to get rid of any pulps. Add to the icing sugar spoon by spoon, stirring in between each spoon. If you feel like two tablespoons are enough, there's no need to add the whole amount of orange juice. Stir to combine to get a nice sugary glaze. The glaze should be thick but still pourable. Set aside for 5 minutes for the sugar to dissolve in orange juice. Then spread evenly over the baked but cool orange cake.TipStart pouring the glaze from the middle to the outer corners of the cake to create a more even glaze.
Decorate your easy orange cake with candied orange slices, cut on even slices and serve. Store covered at room temperature for up to three days.