in glass casserole pan, melt 1/2 cup butter in preheating oven. add brown sugar and mix. arrange pineapple slices to cover bottom of skillet. set aside.
combine flour, baking powder, and salt in a small bowl.
separate 4 eggs with the whites in a large bowl. beat egg whites just until soft peaks form. add white sugar gradually. beat until medium-stiff peaks form.
beat egg yolks at high speed until very thick and yellow.
with rubber scraper, gently fold egg yolks and flour mixture into whites until blended.
fold in 1 tablespoon melted butter or margarine and almond extract.
spread batter evenly over pineapple.
bake until surface springs back when gently pressed with fingertip and a toothpick inserted in the center comes out clean, about 30 to 35 minutes. Cool for 5 minutes, loosen the edges, invert onto serving plate.