Place sliced butter ( butter is sliced so that it melts faster and evenly) in a saucepan over low heat. Melt the butter until it is foamy on the surface, swirling occasionally. The butter will have a nutty aroma and brown specks will form at the bottom of the pan. Remove from heat and pour into a small bowl with all the bits. Leave it to cool to room temperature before using.
Sift flour, cornstarch, baking soda and salt in a large mixing bowl. Set aside.
In a large mixing bowl, add brown butter, brown sugar and granulated sugar and beat with a whisk or hand mixer until combined, about a minute.
Mix in egg and vanilla until well blended.
Add the flour mixture to the butter mixture and use a spatula to mix until just combined.
Fold in butterscotch, chocolate chips and chunks.
Cover the cookie dough and chill in the fridge for at least 2 hours or overnight.
Take the dough out of the fridge and leave to soften to room temperature for 10 minutes
Pre-heat oven to 160°C Fan/180°C/350°F. Line 2 baking tray with parchment paper.
Scoop two tablespoons each of the cookie batter and roll into balls. Place cookie balls on the baking tray at least 2 inches apart. If desired, press a few butterscotch chips, and chocolate chips on top of the cookie dough balls.
Bake for 10 – 12 minutes. Remove from oven and place on cooling rack. The centre of cookies may look slightly under-baked. Leave to cool for at least 10 minutes before serving.