Grease a 23x33cm baking tin and preheat oven to 180 C.
Drain crushed pineapple and reserve juice. Combine melted butter, brown sugar, pineapple and 2 tablespoons pineapple juice. Spread onto the bottom of the greased baking tin. Add enough water to remaining juice to make 325ml of liquid. Set aside.
Combine flour and baking powder. Set aside.
Cream butter and caster sugar until light and fluffy. Add eggs and vanilla and beat for 1 minute. Add flour mixture alternately with juice, beginning and ending with the dry ingredients.
Pour batter over pineapple mixture in baking tin. Bake in the preheated oven for 40 minutes, or until skewer inserted in centre comes out clean. Cool in tin 5 minutes and then invert onto a serving platter.