Drain the canned pineapple, reserving 80ml of the pineapple juice. Refrigerate the pineapple pieces until needed.
In a medium bowl, mix together melted butter and sugar until well blended. Add milk, eggs, vanilla, and reserved pineapple juice.
In a separate large bowl, combine oats, sea salt, and baking powder.
Gently mix the dry mixture into the wet mixture and stir until completely blended. Pour into a lightly greased 8×8 inch baking dish. Cover with plastic wrap and refrigerate overnight.
When ready to bake, preheat the oven to 180°c while you prepare the topping. In a small bowl, mix together melted butter, brown sugar, and crushed pineapple pieces.
Spread the topping mixture evenly over the cake. Bake for about 25 minutes, or until the cake feels slightly firm to touch. Turn the oven to the grill setting. Grill the cake for about 1 minute, or until the topping starts to evenly brown and caramelize. Watch closely, as the sugar topping will burn quickly if not careful.
Remove cake from oven and let cool for at least 15 minutes. It will become more set, like a cake, as it sits. Keeps well in the refrigerator and can be eaten for several days.
