Preheat an oven to 350°F. In a 12-inch cast iron skillet cook the butter, sugar and salt, stirring constantly, until the sugar dissolves and the entire mixture is bubbling thickly. Remove from heat.
Sprinkle 1 teaspoon of the Korean red pepper flakes over the caramel. Arrange the pineapple rings on top of the caramel, cutting apart rings to fit them in between the whole rings. You may not use the entire pineapple, reserve the leftovers for another use or just eat them. Sprinkle the remaining pepper flakes over the pineapple.
Stir the yogurt and the lime juice together then add it to the batter. Mix to incorporate, then add the flour and stir until the flour is absorbed.
Pour the batter over the pineapple and then carefully spread it to the edges of the pan evenly. Place in the oven and bake 60 to 70 minutes, or until a toothpick inserted in the middle of the cake comes out clean.
Korean red pepper flakes are a fruity spicy pepper flake that can be found in large bags at Asian or Korean grocery stores or online. They come in various spicy strength, from mild to spicy. This recipe uses the mild version (deolmaewoon gochugaru) but if you only have the spicy version (maewoon gochugaru), cut the pepper flakes down to 1 teaspoon. Aleppo pepper is also a great substitute for Korean red pepper flakes, use the same amount if you have that on hand instead. Or you can substitute 1/2 teaspoon of regular red pepper flakes, along with 1/2 teaspoon of fresh ground black pepper and 1/2 teaspoon of sweet paprika.
Adapted from a recipe in the cookbook Hartwood by Eric Werner and Mya Henry