Prepare your pineapple- set aside.
Butter or grease well a 9" round cake pan.
Preheat the oven to 350 degrees.
In medium bowl whisk together flour, baking powder & salt- set aside.
In a small saucepan, combine 4 tbsp of the butter & the brown sugar.
Bring to a simmer- stirring constantly until mixture is slightly darkened & thick.
Pour into prepared pan & spread out.
Add the prepared pineapple in a single layer in the pan.
Sprinkle generously with more brown sugar.
In a stand mixer, beat the remaining butter with the granulated sugar on med-high until fluffy. Scrape down bowl as necessary.
Beat in eggs one at a time.
Beat in vanilla.
Reduce the mixer to low- add flour mixture. Beat until combined.
Carefully spread batter over the pineapple & bake until toothpick inserted in center comes out clean- approx 55-60 min.
Let cool at least 10-15 min before inverting on serving platter.