Remove butter from the fridge at least 30 minutes beforehand. Mix until you get a kind of paste.
Preheat oven to 180°C.
Add sugar and egg yolk. Mix.
Pour half of mandarin juice (1 dl.), 1,5 teaspoons of blossom flower and 2 teaspoons of cointreau. Mix.
Add flour, zest and baking powder.
Beat egg white until foamy and incorporate to the mix.
Butter mold, pour preparation and bake for 25 minutes.
In the meantime, make the sirup. Bring to a boil the rest of the mandarin juice (1 dl.) with powdered sugar, 1 tablespoon of blossom flower and 2 tablespoons of cointreau.
Pour on top of the warm cake and leave to cool.
The cake will absorbe all the liquid.
