Start by getting your pan ready—this is the part that makes the whole thing work. Prep the topping: Melt the butter in a small saucepan or the microwave, then pour it into your 9-inch round cake pan. Tilt the pan to coat the bottom evenly. Sprinkle the brown sugar over the melted butter—don't stir, just let it sit. Arrange the 7 pineapple rings on top of the sugar. Place one cherry in the center of each ring and one in the center of the pan if there's room.
Preheat: Set your oven to 350°F (175°C) and make sure a rack is in the middle. While it heats, whisk together the flour, baking powder, and salt in a medium bowl. Set aside.
Cream butter and sugar: In a large bowl, beat the softened butter and sugar with an electric mixer on medium speed until the mixture looks pale and fluffy—about 2 to 3 minutes. Scrape down the bowl once during mixing. This step builds the cake's structure.
Add eggs and vanilla: Beat in the eggs one at a time, making sure each is fully incorporated before adding the next. Stir in the vanilla. The batter should look smooth and slightly glossy at this point.
Alternate dry and wet: Turn the mixer to low and add the flour mixture in three additions, alternating with the milk—start and end with flour. Mix just until the last streak of flour disappears. Overmixing makes the cake tough.
Pour and spread: Spoon the batter over the pineapple layer, being careful not to disturb the fruit. Use your spatula to spread it evenly to the edges. The batter will be thick—that's correct.
Bake: Slide the pan into the oven and bake for 40 to 45 minutes, or until the top is golden and a toothpick inserted into the center of the cake comes out clean. You may see a few moist crumbs but no wet batter. The edges should just begin to pull away from the pan. The smell of caramelized sugar and vanilla will fill your kitchen.
Cool and invert: Let the cake cool in the pan on a wire rack for exactly 5 minutes. Place your serving plate upside down over the pan, then flip the whole thing over in one quick motion. Lift the pan off carefully. If any sugar sticks, just scrape it onto the cake. Serve warm or at room temperature.