Sprinkle the yeast over 1/2 cup of warm water in a small bowl. The water should be no more than 100 degrees F (40 degrees C). Let stand for 5 minutes until the yeast softens and begins to form a creamy foam.
Beat egg, melted butter, vanilla extract, and milk together in a large bowl, until smooth. Sift cake flour, cornmeal, sugar, and salt together in a small bowl, then stir the flour mixture into the egg mixture, mixing until just combined. Stir in the yeast mixture until smooth. Cover with a light cloth and let rise in a warm place (80 to 95 degrees F (27 to 35 degrees C) until doubled in volume, 1 to 2 hours. Pour mixture into a 9 inch round pan lined with parchment paper. Sprinkle with raisins.
Place a steamer insert into a saucepan, and fill with water to just below the bottom of the steamer. Cover, and bring the water to a boil over high heat. Add the cake, recover, and steam until a toothpick inserted into the center comes out clean, about 25 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack.