In a microwave-safe measuring cup, microwave butter on high until melted. Continue heating butter on high in 30-second increments, stirring occasionally, until butter is amber-colored and has a nutty aroma, 2 to 3 minutes more. Refrigerate butter until solidified but spreadable, about 30 minutes.
Preheat oven to 350°.
In a large bowl, beat softened brown butter and brown sugar with a mixer at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating well after each addition. Beat in sweet potato, buttermilk, and maple syrup until well combined.
In a small bowl, whisk together flour, baking powder, cinnamon, salt, nutmeg, and cloves. With mixer on low speed, gradually beat flour mixture into sweet potato mixture just until combined, stopping to scrape bowl.
For topping:
In a 10-inch cast-iron skillet, melt butter over medium heat. Sprinkle brown sugar into melted butter (do not stir together). Top with pecans, then pineapple (do not stir together). Gently spread batter onto topping.
Bake until a wooden pick inserted in center of cake comes out clean, 40 to 45 minutes. Let cool on a wire rack for 10 minutes. Carefully invert skillet onto a serving plate (do not remove skillet), and let stand for 2 to 3 minutes for topping to settle. Remove skillet, and let cake cool for at least 15 minutes before serving. Store in an airtight container for up to 3 days.
Recipe by Southern Cast Iron at https://southerncastiron.com/pineapple-sweet-potato-upside-cake/