Heat oven to 350 degrees F.
Onto sheet of waxed paper, sift together all-purpose flour, cake flour, rice flour, baking powder, salt and nutmeg.
In large bowl of freestanding mixer, on low speed, beat butter until creamy, 2 minutes. Beat in vanilla bean seed scrapings. Blend in superfine sugar and vanilla. Beat on low speed 2 minutes. Add sifted flour mixture in 2 additions, beating slowly until particles of flour are absorbed and smooth and a soft dough forms. Scrape down sides of bowl once or twice to keep batter even textured.
Put dough in 9-1/2-inch round tart pan with removable bottom; press out evenly. Prick in 15 random places.
Bake in 350 degree F oven 30 minutes, until shortbread is set and turns medium tan. Immediately sprinkle with superfine sugar. Let cool in pan on rack 10 minutes.
Carefully unmold shortbread, leaving it on round pan base. After 10 to 15 minutes, cut into wedges and let cool completely. Store in airtight tin at room temperature for up to 1 week. Makes one 9-1/2-inch shortbread, or 12 pieces.