Preheat your oven to 350°F (175°C).

In a saucepan over medium heat, melt ¼ cup (½ stick) of the butter. Once melted, add the brown sugar and stir until dissolved and bubbly.

Pour this mixture into the bottom of the greased 9-inch round cake pan.

Arrange the pineapple slices on top of the brown sugar mixture in a single layer. Reserve the juice from the can for later use.

In a mixing bowl, cream together the remaining ¼ cup (½ stick) butter and granulated sugar using an electric mixer or whisk until light and fluffy.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In another bowl, combine the flour, baking powder, and salt. Gradually add this dry mixture to the wet mixture, alternating with the milk and mixing until just combined.

Pour the batter evenly over the pineapple slices in the cake pan, smoothing the top with a rubber spatula.

Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted in the center comes out clean.

Once baked, remove the cake from the oven and let it cool for about 10 minutes. Then, carefully invert the cake onto a serving plate.

Allow it to cool slightly before serving.