Whisk softened butter with sugar and salt.
Add egg and mix.
Add flour and knead (with hands).
Place in fridge for 1 hour.
Add poppy seed and let boil for 2 minutes.
Pour in rum and leave to cool.
Roll out the dough in a rectangular form (on floured surface).
Spread the filling on top and roll up.
Divide the roll in half and wrap in cling film.
Place in the freezer for about 1 hour.
Cut in slices (1/2 cm) and put on baking tray (with baking paper).
Bake in preheated oven at 180° C for about 14 minutes.
Mix icing sugar with water and draw lines over the poppy seed biscuits. This works well with a freezer bag.
Leave to cool and store in a biscuit tin.