Preheat your oven to 180°C (160°C fan). Lightly butter or line two baking trays.
In a bowl, beat the butter with an electric whisk until softened. Gradually add the sugar, continuing to beat until combined.
Mix in the flour and vanilla extract (if using) to form a dough.
Shape the dough into 16 walnut-sized balls and place them on the prepared baking trays, leaving about 5 cm of space between each.
Dip a fork in water and use it to flatten the biscuits.
Bake in the preheated oven for 15-20 minutes until the biscuits are a very pale golden color.
Remove from the oven and transfer the biscuits to a wire rack to cool completely.
If desired, melt the chocolate in a microwave-safe bowl and drizzle it over the cooled biscuits. Allow the chocolate to set for about 30 minutes.