Butter a 9" round cake pan. Pre-heat oven to 350 degrees.
In small sauce pan, melt butter and brown sugar together over medium heat.
Stir until sugar dissolves and then stop stirring.
Allow mixture to just start to bubble. This only takes a few minutes.
Pour sugar/butter mixture in to bottom of prepared cake pan. Arrange pineapple slices in mixture and set aside.
In a large bowl, whisk together flour, baking powder and salt. Set aside.
In a stand mixer, beat together sugar and butter until light and fluffy, about 7 minutes.
Add vanilla and beat to incorporate.
Add egg yolks, one at a time, beating after each addition.
Add flour mixture in 3 additions, alternating with milk, ending with flour. Scrape bowl down as needed.
Transfer batter to the large bowl that held the flour mixture.
Clean and dry mixer bowl thoroughly.
Place egg whites and cream of tartar in mixer bowl. Whip until stiff peaks form (at least 10 minutes).
Gently fold egg whites in two batches to the cake batter.
Pour cake batter over pineapple slices in the cake pan.
Bake for 35-55 minutes, checking the cake at 25 minutes for doneness.
Cake is done when top is golden and toothpick comes out clean.