Begin by beating together butter and sugar along with vanilla in a large mixing bowl until they are light, creamy and fluffy.
You can use your electric beater or stand mixer or a wired whisk for this. Add vanilla and beat together. Using a large bowl cream them well by scrapping the sides as required.
Sift flour custard powder, salt (if using unsalted butter) together in a separate bowl.
Add the sifted dry ingredients to the butter sugar mixture and mix well with a wooden spoon.
Bring the dough together by adding few 3-4 teaspoons of milk if required. Cover the dough well and refrigerate for 30 minutes to 3 hours.
When you are ready to bake the cookies, preheat your oven at 180C or 350 F. Grease 2 large baking trays with some butter or cooking spray. Alternately you can line the cookie sheets with parchment paper.
Roll the dough into a circle of 1/4 inch thickness. Cutout cookies using a round or any other shaped cookie cutter.
Arrange in the greased baking tray and prick some holes on the top. I have used two different cutters for 2 batches
Bake in the preheated oven for 10-12 minutes. The cookies will slightly change color. Go by the flavor as these cookies will be soft on removing from oven and turn crispy on cooling. Cool the cookies in a wire rack thoroughly before filling with buttercream. To make custard creams filling, combine soft salted butter, sugar, vanilla and thick mango pulp in a bowl. Using a electric beater or handheld one, beat until light and creamy.
To assemble the biscuits simply spread some cream on a cookie with a piping bag or palette knife and close with another cookie.