Preheat the oven to 180°C (350°F/Gas 4). Grease 2 large baking trays.
In a small bowl using an electric mixer, beat the butter and sugars until light and creamy. Beat in the egg and vanilla. Transfer the mixture to a large bowl. Sift the flour and bicarbonate of soda over the mixture, stir to combine, then stir in the chocolate. Roll level tablespoons of dough into balls and place 2.5 cm (1 inch) apart on the prepared trays.
Bake for about 12 minutes. Cool briefly on the trays, then transfer to wire racks.