Preheat the oven to 180ºC. Lightly grease a 20cm x 30cm shallow tin and line the base and two sides with baking paper.
Cream the butter and sugar until light and fluffy, then add the egg yolks and mix thoroughly.
Add the lemon juice and then sift in the flour and baking powder and mix to a firm dough.
Press the dough evenly into the prepared tin, and spread over the jam. You don’t need a thick layer.
Beat the egg whites until stiff then gently fold in the caster sugar and the coconut using a metal spoon. Spread carefully over the jam, again trying to keep an even thickness. Sprinkle with a little more coconut.
Bake for about 25 minutes until the coconut is just turning golden brown. Remove from the oven, and cut into squares or fingers while it is still warm. Cool in the tin on a wire rack.