Whisk the butter, icing sugar and salt until frothy and the mixture forms little peaks. Mix in the rum. Add the flour and bind everything together quickly to form a dough. Stand aside to cool for 30 minutes wrapped in cling film.
From the dough, make approximately 50 round cookie shapes and place these at least 2.5 cm apart on a baking tray covered with grease-proof paper. Put aside to cool for 10 minutes. Preheat the oven to 180 °C.
Bake the cookies in the bottom half of the oven for 15–17 minutes. The bottoms can get slightly brown, the tops should still be light in colour. Allow to cool on a cooling rack.
For the filling, melt the chocolate and the butter in a bowl over the hot bain marie. Stir and pour into a litre measure. Allow to cool a little. Use the mixer to beat it into a frothy light brown crème. Smooth this mixture onto the base of one of the baked cookies, and cover with a second cookie. Allow to set on a piece of grease-proof paper.
HINT. If kept sealed and in a cool place sweet kisses will keep fresh for about a week.