Preheat oven to 350 degrees.
Arrange pineapple slices on bottom of 9-inch cake pan.
For topping, melt 4 tablespoons unsalted butter in small pan over medium heat. When melted, add ¾ cup packed dark brown sugar and simmer, stirring constantly, until thick and fully combined (about 5 minutes). Pour completed caramel mixture over pineapple slices and smooth with a spatula until evenly spread over pineapple. Sprinkle 1 tablespoon coarse sea salt granules evenly over caramel.
Sift flour, baking powder and table salt together and set aside.
Beat butter and granulated sugar together until light and fluffy. Add eggs and vanilla and beat until fully combined. Add pineapple juice and gently stir until fully incorporated.
Pour flour mixture into pineapple juice mixture and stir gently until just combined. Pour batter over caramel in cake pan.
Bake until golden and a tester comes out clean, 35-40 minutes.
Remove from oven and allow to cool in cake pan for 5 minutes. Invert over a plate and serve when ready.