Preheat oven to 180C/350F. Grease three oven trays.
Beat butter, vanilla, sugars and egg in a small bowl with an electric mixer until light and fluffy. Transfer mixture to a large bowl; stir in sifted flour and bi-carb soda, in two batches. Stir in chocolate; cover and refrigerate for 1 hour.
Roll level tablespoons of mixture onto balls; place about 3cm (1 1/4 inches) apart on trays. Bake for about 12 minutes, then cool on the trays.