Melt your butter in a small pot or a frying pan over medium heat. Stir occasionally to make sure your butter is browning evenly. Once your butter has changed from yellow to golden brown, remove from the pan and cool down over a bowl of ice. Mix with a spatula to cool down and set aside.
to make the cookies:
Sift flour, baking soda, baking powder and salt together. Set the mixture aside.
In a bowl, beat the cooled brown butter with the brown sugar and demerara sugar until fluffy and light (you can use a wooden spoon, but a mixer will be easier). Add the egg followed by the scraped-out contents of half a vanilla bean, then beat again to incorporate. Slowly add the sifted flour mixture and scrape the bowl down. Add the chopped Tony's and mix just to incorporate.
Using an ice cream scoop (or a spoon is also fine) place cookie dough balls on a lined baking tray with grease proof paper. Chill on a baking tray for 5 to 10 minutes. The longer it chills, the better. Bake at 180c for about 12 minutes and voila! You've got yourself some delicious cookies.