Bianca, if you are measuring by volume, instead of weighing, then you sift the cake flour directly into the measuring cup, without tapping or touching the cup in any way, the flour just floats down undisturbed to fill the cup. All you do is gently level it, without jostling or tapping or shaking. If you are measuring by volume first, then sifting, you are getting way too much flour, which can definitely cause texture problems. Usually this would make the cake dry, but maybe that wasn’t as noticeable with the syrup.
If you are weighing, then you can sift whenever you like.
As for the sugar, Rose recommends making your own superfine in the food processor, you can use your raw cane sugar to do this, no problems.
This cake taught me that the combination of Grand Marnier and dark chocolate is better than either by itself!