3/4cupmini bitter-sweet chocolate chipsfor sprinkling on top of the cake Icing sugar, for sifting on top of the cake
Preheat oven to 175 deg C.
Lightly grease a 9X4 inch baking pan, then line baking sheet around the interior of pan.
To make the topping:
Thoroughly mix the flour and sugar in a large bowl. Drop in chunks of butter, using a table knife, cut the butter into the flour until it reduces to the size of large pearls. Sprinkle vanilla essence over, working with fingertips, knead the mixture together until moist, clumpy lumps are formed. DO NOT overmix.
Mix the batter:
Sift the plain flour, cake flour, baking powder and baking soda. Then toss the chocolate chips with 1 tablespoon of the sifted mixture in a small bowl.
Cream the butter in the bowl of a stand mixer for about 3 minutes.
Add in orange zest, then add the sugar in 3 additions, beating for 1 minute after each portion is added.
Add in egg and beat for 45 seconds.
Mix in the vanilla essence.
Alternately add in the sifted mixture in 3 additions with the buttermilk in 2 additions, beginning and ending with the sifted mixture. Scrape down the sides of the mixing bowl to ensure even mixing.
Stir in chocolate chips.
Spoon the batter into the baking pan, and spread evenly, smooth the top with rubber spatula.
Sprinkle the streusel topping evenly on the cake batter, make sure that the whole surface is covered.
Bake the cake for 1 hour or until a toothpick inserted in the center of the cake turns out clean. The baked cake should pull away slightly from the sides of the pan.
Remove the pan from the oven and sprinkle the chocolate chips evenly on top of the baked cake immediately.
Let the cake cool completely. Peel off the baking sheet from the cake, then sift some icing sugar on top of the cake. Serve.