Method: Combine the plain flour, corn flour, custard powder and baking powder and sift through a sieve. Set aside. In a large mixing bowl, use an electric mixer to beat the butter and icing sugar until fluffy and pale. Mix in the vanilla extract. Sift in the flour mixture. Beat at low speed with an electric mixer. Remember to scrape down the sides of the bowl with a spatula. Lightly knead into a dough. Wrap with a plastic film and roll into a tube in 3½cm thickness. Wrap it up and refrigerate for about 15 to 20 minutes, or until it’s workable. Preheat oven to 180C / 356F. Line two baking trays with baking paper. Remove the dough from refrigerator. Slice into 1½cm-thick portions. Place each portion in single layer on the lined trays, each at least 3cm apart. Use a lightly greased fork to gently press the small dough. Bake in preheated oven, for about 15 minutes, or until lightly brown. Remove from oven and let them sit for 5 minutes. Transfer to a wire rack until completely cool down.