Beat the softened butter the in a large bowl, using a handheld electric mixer at high speed, for about half a minute till fluffy. Mix in the sugar, baking powder, custard powder, vanilla extract and salt until combined. Scrape down the side of the bowl while mixing.
Stir in, with a wooden spoon, the flour in two or three lots but donu2019t overwork the dough. The dough should come together, be moist but not sticky. Add a little flour at a time if the dough feels sticky. Too much flour will result in tough textured cookies.
Divide the dough into half and mix in the cocoa powder into one half. Wrap both the vanilla and chocolate dough in cling wrap and refrigerate for at least 3 to 4 hours so that the dough becomes easy to roll out. If you refrigerate the dough overnight, take it out and let sit at room temperature for a little while so it becomes easy to roll out.
Take the vanilla half of the dough and flatten it out a bit with your palm. Then place it between two sheets of parchment paper and evenly roll it out to 1/8u201d thickness. Using a 2u201d or 2 1/4" square fluted cutter cut out about 15 squares, re-rolling the scraps. Refrigerate the dough for about half an hour if the dough becomes too soft.
Cut out another 15 shapes using smaller hors d'oeuvre cutters or mini cookie cutters. Place them on parchment lined baking sheets and bake them at 190C (375F) for about 15 minutes or so until the cookies start turning light brown at the edges. Let them cool completely on racks.
Repeat with the chocolate cookie dough making a total of 30 cookies (15 square ones and 15 smaller ones) from the dough.
Once the cookies have cooled, take one light coloured mini vanilla cookie and apply melted chocolate on the underside of it. Stick it to the centre of the top of a square chocolate cookie. Reeat with all the similar cookies and then similarly stick all the mini chocolate cookies to the square vanilla cookies.
Once done you will have 30 double coloured Christmas cookies. Store the cookies in an airtight tin.