For the pineapple: In a 10-inch cast-iron skillet over medium heat, melt the butter and add the brown sugar. Cook, stirring constantly, until the sugar melts, about 2 minutes. Remove from the heat and arrange the pineapple slices in the skillet. Fill in the spaces with the crushed pineapple. Set aside.
For the cake: Meanwhile, combine the cake ingredients in this fashion: In one bowl, whisk together the flour, baking powder and salt. In bowl number 2, combine the milk, vanilla extract and 1/4 cup of the reserved pineapple juice.
In the bowl of a stand mixer fitted with a paddle attachment, combine the granulated sugar and coconut oil on medium speed until thick and creamy, about 3 minutes. Beat in the egg. Add half of the flour mixture and mix on low until just combined. Add the milk mixture and stir until just combined. Add the rest of the flour and mix until combined, about 1 minute.
Pour the batter over the pineapple slices in the skillet and spread evenly. Bake until the top is golden brown and a toothpick comes out clean when inserted, 25 to 35 minutes, checking after 20 minutes to make sure the top isn't getting too brown. If it is, cover loosely with aluminum foil.
Let cool for 15 minutes. Cover with a cutting board or large plate and invert the cake. Ta daaaaaaa, you are ready to eat!