Line the bottom of the pan with parchment paper. Preheat oven at 350 F (180 C) for conventional oven (bottom heat). For convection oven, you can try 320 F (160 C)
Peel one Mandarin orange. Remove the skin and then cut into small chunks. Set aside. Squeeze out about 50 ml of juice from another fresh Mandarin orange
Prepare cake batter:
Mix the flour, salt, and baking powder. Set aside
Make sure the butter is soft but not melty. You can easily push it with your finger. If it's melty, put it in the fridge for a bit but not too long that it becomes cold solid again
Crack the eggs in a bowl and whisk them. Set aside. It's better to do this than cracking an egg one by one into the cake batter later. The eggs incorporate into the batter much better
Cream the butter and sugar until creamy, light, and fluffy with a stand mixer or hand mixer over high speed. The color turns pale too. Then slowly add 1/3 of the egg and continue to beat until it's mixed before adding the next 1/3 until you run out of the egg
Add the orange zest. Sift in 1/3 of the flour mixture and mix over low speed and continue to add until you run out of the flour mixture. It's okay if it's not thoroughly combined yet at this point
Add the orange juice into the cake batter and mix over low speed again to combine. Fold in the fresh orange cubes you cut earlier with a rubber spatula and if you still see any loose flour, you can gently fold it in until combined
Pour the batter into the prepared pan. Decorate the top with orange segments, about 10-12 slices are good
Baking:
Place the cake pan in the middle rack. Bake for 50-55 minutes or until golden brown and a cake tester inserted comes out clean. The time is just for reference. Your oven may need shorter or longer time. You can start checking at about 50-minute mark
Serving:
The butter cake is not meant to be served on the same day. I suggest let the cake cools down completely and then wrap it up with a cling wrap and keep at room temperature for 24 hours before serving (trust me, it worth the wait). The cake will be moist and develop a full flavor after 24 hours and longer