Preheat oven to 180oC/160oC fan-forced. Grease a 3cm-deep, 20cm x 30cm (base) lamington pan. Line with baking paper, allowing 2cm overhang on all sides.
Using an electric mixer, beat butter and 1/2 cup of the sugar until light and fluffy. Add 1 egg and beat until combined. Sift flour and baking powder over butter mixture and stir to combine. Stir in buttermilk. Press mixture over base of the prepared pan. Spread jam over mixture.
Whisk remaining eggs and sugar together in a bowl. Stir in the coconut. Sprinkle mixture over jam. Bake for 35 to 40 minutes or until light and golden. Cool in pan. Cut into pieces. Serve.