Grease 2 large baking trays. Preheat oven to 180 degrees C.
Sift the flour and bicarb 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 2.5cm apart. Bake for about 12 minutes.
Cool cookies briefly on trays. Transfer to a wire rack to cool.
I have just finished making these and they are fantastic. I weighed the ingredients as there is a differenc between the grams stated and the cup measurements ... Doing this resulted in perfect cookies that are crisp on the edges and chewy in the middle. The kids loved them so I'll be making these again for sure! - 15 Oct 2010