Preheat oven to 350 degrees F. Put 2 tablespoons of the butter in a 6 inch round cake pan. Set it in the oven, set the timer for 3 minutes, and let the butter melt. Keep a close eye on it!
Remove pan with melted butter from the oven and add the brown sugar. Stir until blended, then spread evenly across bottom of the pan.
Cover bottom over the pan with pineapple chunks.
With an electric mixer, beat the remaining 4 tablespoons of butter and the 1/3 cup of granulated sugar until creamy. Beat in the vanilla. Add the egg and stir util mixed, then beat for 30 seconds.
Mix together the flour, baking powder and salt. Add half the mixture to the batter and stir well. Stir in the milk, then add remaining flour mixture and stir until smooth.
Scrape batter into the pan so that it covers the pineapple. Smooth the top and gently tap the pan on the counter to settle the batter. Bake until a toothpick inserted comes out with clean crumbs – about 50 minutes (check at 45).
Let cool in the pan for 10 minutes, then carefully loosen from the side of the pan. Place a serving plate over the top of the cake pan, invert the cake and let sit until cake releases itself from the pan. Do not shake or tap the pan (like I did!) or you might break the cake. It is very delicate at this point.