Drain the canned pineapple, reserving ⅓ cup of the pineapple juice. Refrigerate the pineapple pieces until needed.

In a medium bowl, mix together ½ cup melted butter and ⅓ cup sugar until well blended. Add milk, eggs, vanilla, and reserved ⅓ cup pineapple juice.

In a separate large bowl, combine oats, sea salt, and baking powder.

Gently mix dry mixture into wet mixture and stir until completely blended. Pour into a lightly greased 8x8-inch baking dish. Cover with plastic wrap and refrigerate overnight.

When ready to bake, preheat the oven to 350°F 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 broil setting. Broil 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.