Preheat oven to 180C/350F. Grease a 20cm x 30cm (8-inch x 12-inch) (top measurement) lamington pan; line base and long sides with baking paper, extending the paper 5cm (2 inches) over the sides.
Make coconut crumble. Combine sifted flour and sugar in a small bowl; rub in butter. Stir in coconut flakes.
Beat butter and sugar in a small bowl with an electric mixer until light and fluffy. Beat in eggs, one at a time. Fold in sifted flour, baking powder, sour cream and milk. Spread the mixture in the pan.
Meanwhile, drain pineapple well; discard juice (or reserve for another use). Cut pineapple slices in half horizontally, then in half crossways.
Arrange pineapple slices on mixture in the pan; sprinkle with coconut crumble.
Bake slice for about 45 minutes. Stand slice in pan for 10 minutes; transfer to a wire rack to cool.