Preheat your oven to 350°F (175°C). Melt 1/2 cup (1 stick) of butter in a 9-inch round cake pan in the oven, or in the microwave. Spread the melted butter evenly over the bottom of the pan. Sprinkle the brown sugar evenly over the melted butter.
Arrange the pineapple slices in a single layer over the brown sugar mixture. You can place a maraschino cherry half in the center of each pineapple ring for a festive look.
In a medium bowl, whisk together the flour, baking powder, and salt. In a separate large bowl, cream together the softened butter and granulated sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined. Be careful not to overmix.
Pour the cake batter evenly over the pineapple slices in the cake pan. Spread the batter gently to cover the entire surface.
Bake in the preheated oven for 45-50 minutes, or until a wooden skewer inserted into the center comes out clean. The top should be golden brown.
Let the cake cool in the pan for 10 minutes. This allows the caramel to set slightly. Place a serving plate over the top of the cake pan, then carefully invert the cake onto the plate. If any pineapple slices stick to the pan, gently remove them and place them back on the cake.