Preheat the oven to 180°C.
Cream the butter, vanilla and sugar until smooth using an electric beater. Add the egg and beat to incorporate.
Fold in the flour and add the chocolate drops.
Line a baking tray with baking paper. Roll 2 T mixture into a ball, add more chocolate drops and press into the surface of the cookie. Repeat with the remaining mixture and place on the tray, leaving enough space between them as they spread while baking.
Bake for 15–20 minutes, then cool.