Fold in sifted cake flour, baking powder, cocoa powder, and salt, and mix well with a spatula. Scrape down the mixture from the side of the mixing bowl, and combine well until a smooth batter forms. Set aside.
Beat the softened cream cheese with a stand mixer or electronic hand mixer until smooth. Add sugar, egg and corn flour, and continue beating the mixture until smooth and no clumps remain in the filling. Set aside.