Preheat the oven to 350 degrees F.
For the fruit layer, melt the butter in a 9 or 10 inch cast iron skillet. Add the agave and lemon juice and bring to a low boil for about 5 minutes. Sprinkle brown sugar over the top of the syrup. Place the pineapple on top of the mixture in a decorative pattern and set aside to cool.
Sift together the cake flour, Clabber Girl Baking Powder and salt. Set aside. Beat the egg whites until soft peaks form. Slowly pour in the agave or pineapple juice concentrate, and beat to shiny peaks. Whisk the egg yolks until pale yellow then stir in the melted butter and vanilla. Gently stir the yolk mixture into the egg whites and fold in the flour mixture in 4 equal parts.
Pour the batter evenly over the pineapples (they tend to rise into the batter otherwise) and smooth the top. Bake for 35 to 40 minutes, until a toothpick inserted in the center of the cake comes out clean. Cool on a rack for 15 minutes. Run a knife around the edge of the pan and invert the cake onto a serving platter. Let stand for a few minutes for the cake to absorb the juices. Remove the skillet, cool and serve.