Line 2 flat cookie sheets/trays with parchment/baking paper.
Place butter, brown sugar and white sugar into the bowl of the mixer or bowl for hand mixer and beat until light and creamy.
Add in egg and vanilla and beat till combined.
Add in flour, bicarb soda, salt and corn starch/flour and beat until mixed through.
Chop up Toblerone bars into chip sized pieces. (Reserve 12 of the bigger pieces to place on the cookies before baking)
Place the chopped bars into the dough and mix through with a spatula or spoon until the chocolate is dispersed throughout the dough.
Take 2 tablespoons of mixture and roll into balls. Alternatively, use a 2 tablespoon cookie scoop.
Place onto prepared cookie sheets leaving about 3 inches between each cookie to allow for spreading. I fitted 6 on each tray. Place the reserved pieces on the top of cookies if using.
Place the trays in the freezer for at least 2 hours. If there is limited space in the freezer, place the balls close to each other on one tray. You can seperate them onto 2 trays before baking.
Preheat oven to 180c/350F, 10-15 minutes before they're ready to bake.
Place in the oven for 10-12 minutes.
Cookies are ready when the edges are starting to turn golden in colour and the centres are still soft. The cookies will continue to cook and set on the tray after baking so it's best to take them out before they are baked through.
Cool down completely before serving.