Pour the melted butter into your cake pan and spread it evenly. Sprinkle the brown sugar over the butter, then arrange the pineapple slices on top.
In a bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt.
In another bowl, beat the butter and sugar until light and fluffy.
Add the eggs one at a time, mixing well after each one. Stir in the vanilla.
Add the flour mixture in 3 batches, alternating with the buttermilk. Start and end with the flour mixture. Mix until just combined.
Gently pour the batter over the pineapple layer and smooth the top.
Bake until the cake is golden and a toothpick inserted into the center comes out clean.
Let the cake cool for a few minutes, then carefully turn it out onto a serving plate so the pineapple side is on top.