Preheat oven to 350°F (175°C) and position rack in the center. If using a cast iron skillet, warm it in the oven during preheating.
Melt ½ cup butter in your skillet over medium heat on the stovetop or in the oven. Swirl to coat bottom and sides completely.
Sprinkle ¾ cup brown sugar evenly over the melted butter, pressing gently with the back of a spoon to create an even layer.
Drain pineapple rings thoroughly, pat dry with paper towels, and reserve ⅓ cup of juice. Pat maraschino cherries dry as well.
Arrange 7-9 pineapple rings in a single layer over the brown sugar. Cut remaining rings in half to fill gaps around the edges.
Place one cherry in the center of each pineapple ring. Tuck additional cherries into spaces between rings. Sprinkle with pecans if using.
In a medium bowl, whisk together flour, baking powder, and salt for 30 seconds to ensure even distribution.
In a large bowl, beat softened butter and granulated sugar on medium speed for 3-4 minutes until light and fluffy.
Add eggs one at a time, beating well after each addition. Mix in vanilla extract. The mixture may look slightly curdled.
Combine reserved pineapple juice and milk in a small measuring cup. Add one-third of the flour mixture to the butter mixture and mix just until combined.
Add half the juice mixture, then another third of the flour, the remaining juice mixture, and finally the remaining flour. Mix just until no streaks remain.
Pour batter evenly over the arranged fruit, spreading gently with a spatula without disturbing the pineapple. Tap pan on counter to release air bubbles.
Bake in preheated oven for 40-50 minutes until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Remove cake from oven and cool in pan on a wire rack for exactly 10 minutes. Do not exceed this time.
Run a butter knife around the inside edge of the pan. Place a serving plate upside down on top of the pan.
Using oven mitts, hold both pan and plate firmly together. In one quick, confident motion, flip both over together.
Let sit for 2-3 minutes, then carefully lift the pan straight up. If any fruit sticks, replace it with a fork.
Allow cake to cool for at least 15-20 minutes before slicing. Serve warm for best flavor.