Preheat oven to 180 °C.
Grease and line a deep, 20 cm round or square cake tin.
Sift flour into a large mixing bowl. Add poppyseeds and make a well in the centre.
Combine butter sugar, jam, rind and juice in a medium pan.
Stir over low heat until butter has melted and sugar has dissolved.
Remove from heat. Add butter mixture to dry ingredients.
Stir until just combined, then add eggs.
Mix well, but do not overbeat.
Pour mixture into prepared tin and smooth surface.
Bake 25-30 minutes or until a skewer comes out clean when inserted in the centre of the cake.
Cover with butter icing if desired, and sprinkle with poppyseeds.
For butter icing: Beat together icing sugar, butter, milk and essence until creamy.