In your 9" cake pan, pour melted butter. Tilt the pan to spread it evenly across the bottom of the pan. Pour in brown sugar and spread evenly over the butter. Lastly, arrange pineapple slices over the brown sugar. Set aside.
In a large bowl, whisk together wet ingredients.
In a medium bowl, combine dry ingredients. Add dry ingredients to wet and mix until just combined.
Pour batter over the pineapple-lined cake pan. Smooth the top. Bake for 30 minutes, then cover top loosely with foil and bake for 10-15 more minutes (or until a toothpick comes out clean.)
Loosen the sides of the cake from the pan with a knife. Invert cake onto plate or serving platter and let stand five minutes. Carefully lift the pan off the cake. Allow to cool completely, then enjoy! Store in the fridge.
Notes
If you're not serving it immediately, let cool completely then store in the fridge for 3-4 days!