Preheat the oven to 350°F. Lightly grease 2 baking pans or line them with parchment paper.
Beat sugar, butter in a stand mixer (using the paddle attachment) or using a hand mixer until light and fluffy.
Add vanilla and egg (or egg replacer mixture) and mix until incorporated.
Sift flour and custard powder into the butter mixture and mix until combined. Stir in chocolate chips (if using).
Roll the dough into small walnut size balls and place them on the prepared baking sheet. Flatten the balls lightly with fingers.
Bake 12~15 minutes or until lightly golden.
Remove the cookies from the oven and place them a cooling rack to cool completely. Store them in an airtight container.