Remove butter of the fridge at least 30 minutes before.
Preheat oven to 180°C.
Mix softened butter with sugar. Add one by one egg yolks while mixing.
Pour half of orange juice, zest, flour and baking powder. Wisk until you get an homogenous paste.
Beat egg white, incorporate in the previous mix.
Butter and divide the mixture among the jars.
Bake for 30-35 minutes.
In the meantimes, heat the rest of orange juice with icing sugar until dissolution. Pour on top of the warm cakes and close the jars. Leave to cool.