Preheat the oven to 350°F (175°C).
Place the 10-inch cast-iron skillet over medium heat and add the vegan butter; once melted, stir in the 1/2 cup dark brown sugar and cook, stirring constantly, until the sugar dissolves and the mixture is glossy, about 2 minutes.
Remove the skillet from heat and arrange the drained sliced pineapple in the caramel, placing maraschino cherries into the center holes if desired; spread the drained crushed pineapple into the gaps and over the top.
In a large mixing bowl, combine the applesauce, canola oil, 1/2 cup granulated sugar, 1/3 cup reserved pineapple juice, and vanilla; mix until combined.
Place a fine-mesh sieve over the bowl and sift together the all-purpose flour, baking powder, and sea salt into the wet ingredients.
Fold the dry ingredients into the wet with a wooden spoon or spatula until just incorporated, taking care not to overmix.
Spread the batter evenly over the pineapple in the skillet.
Bake in the preheated oven for about 35–40 minutes, until the cake is golden on top and a toothpick inserted into the center comes out clean.
Remove the skillet from the oven and let cool for about 10 minutes.
Loosen the cake edges with a butter knife, place a 12-inch (or larger) cake platter upside down over the skillet, and quickly invert the skillet to release the cake onto the platter.
Serve immediately or let cool completely before refrigerating; cake keeps well in the fridge for 2–3 days.