In a mixing bowl, beat together the unsalted butter and superfine sugar until the mixture is light and fluffy, about 3-5 minutes with an electric mixer on medium speed.
Gradually sift in the all-purpose flour and custard powder (or corn starch and vanilla). Add a few drops of vanilla extract. Mix until everything is well combined and forms a soft dough.
Wrap the dough in plastic wrap and refrigerate for at least 30 minutes to help the biscuits hold their shape during baking.
Preheat your oven to 350°F (175°C). Line your baking sheet with parchment paper for easy removal of the biscuits after baking.
Once chilled, remove the dough from the refrigerator. On a lightly floured surface, roll out the dough to about 1/4 inch thickness. Use a cookie cutter to cut out your biscuits. Place each cut-out onto the prepared baking sheet.
Bake the biscuits in the preheated oven for about 10-12 minutes, or until they are lightly golden around the edges. Allow them to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.
While the biscuits are cooling, prepare the filling. In a medium bowl, beat together the room temperature unsalted butter and powdered sugar until smooth. Add the custard powder (or corn starch and vanilla) and a few drops of vanilla extract. Mix until well combined and creamy.
Once the biscuits are completely cool, spread a generous amount of the filling onto the bottom side of one biscuit. Top with another biscuit, bottom side down, and gently press together. Repeat until all biscuits are filled.
Your Custard Creams are now ready to be enjoyed! Serve them with a cup of tea or coffee, and watch them disappear.