Preheat oven to 180℃ and line a 18 x 28cm rectangular lamington slice pan with baking paper.
Using the stand mixer fitted with the paddle attachment, beat the butter and sifted icing sugar on medium speed until light and fluffy.
Reserve 1 cup of the crumble walnut mixture. Press the remaining crumble mixture into the pan and top with frozen berries before sprinkling the reserved crumble mixture over the top.
Bake for approximately 40 minutes then cool in tray before cutting into desired size squares.