Grease 2 large baking trays. Preheat oven to 180ºC.
Sift the flour and bicarbonate of soda onto a plate. Beat the butter and sugar in a small bowl until the mixture is light and fluffy. Add egg and vanilla essence; beat to combine. Transfer mixture to a large bowl. Add the flour mixture, stirring to combine, then stir in the chocolate. Roll level tablespoons of mixture into balls and place on the trays 2.5 cm apart. Bake for about 12 minutes.
Cool cookies briefly on trays. Transfer to a wire rack to cool.