Sift all-purpose flour, cake flour, baking powder, and salt into a bowl and set aside.
Grease an 8-inch cake pan with removable bottom, and then line with parchment paper cut to line the bottom and grease it to prevent the cake from sticking.
Beat 4 egg yolks and 1 cup sugar in an electric mixer with a paddle attachment until doubled in volume. Start adding room temperature butter in four installments while beating until thoroughly combined.
Turn the mixer speed to low and add the flour mixture. Continue beating until cake batter is very smooth and well combined.
Transfer the batter to the prepared cake pan, and smooth the surface with an offset spatula. Refrigerate for 30 minutes.
Meanwhile, beat one egg with 1 tablespoon water to make egg wash. Brush the top of the cake with egg wash. let it rest for 2 minutes and apply egg wash again for a thicker coating.
With the back of a knife decorate all around with cross hatching or whatever pattern you may desire.
Bake in a preheated oven for 40 to 45 minutes, or until golden brown. Cover with foil if browning too quickly. Sliced into wedges to serve.
