Melt butter. Rub the zest into the sugar till it resembles wet sand.
Add eggs to the mixing bowl along with the lemon sugars, mixed until pale and fluffy. Sift cake flour and baking powder. Once the eggs are pale and fluffy, add the rum, pinch of salt and creme fraiche. When all the wet mixture are incorporated, on low speed gently add the dry ingredients in 3 intervals, avoid over mixing. Gently scrap down the sides of the bowl and folded in the few remaining specks of flour. The butter now at room temperature is gently folded in the fluffy mixture, in 3 intervals. At first it looks separated but as you continue folding it the mixture homogenize.
Butter and line 2 loaf pans and divide the batter evenly. Bake this in a preheated 350 degrees oven for 55 to 60 minutes. Check after 30 minutes, because oven varies.
To make it more lemony, while the cakes are baking make the Lemon Simple Syrup .
Notes
Dissolve the sugar in the water in a saucepan and cool until room temperature. Add the lemon juice. Original recipe said that when the cakes are done quickly pour in the lemon simple syrup into each cakes and cool till room temperature. I just brushed the cake lightly with this syrup.