Grease a 6-inch round baking tin. Line the bottom with parchment paper. Set aside.
Combine corn flour, plain flour and baking powder in a small bowl. Set aside.
Combine cocoa powder and boiling water in a medium bowl until a paste is form. Set aside and let cool slightly.
Place the butter and cream cheese in a large mixing bowl and beat with an electric mixer until creamy. Add in the superfine sugar gradually and beat until the batter is light, pale and fluffy. Scrap down the sides occasionally.
Beat in one egg until fully incorporated and scrape down the sides. Beat in the second egg and the vanilla until fully incorporated. Sift in the flour mixture in 2 batches and fold with a large spatula until incorporated. Mix 1/3 of the batter with the chocolate paste.
Drop spoonfuls of the batter into the baking tin. Run a knife / skewer in the batter to create the marbling.
Bake the cake at 160 degrees Celsius for 20 to 25 minutes until a skewer inserted into the center comes out clean. Let the cake cool completely in the pan on a wire rack.