Combine browned butter, brown sugar, and granulated sugar in a stand mixer and mix until all creamed together.
Once the butter and sugars are well combined, add in the egg, vanilla extract, and sourdough discard until all the wet ingredients are fully incorporated.
In a separate bowl, combine the flour, baking soda, salt, and cornstarch and stir until mixed together.
Pour half of the dry ingredient mixture into the wet ingredients and mix together until there are no dry spots. Repeat again with the second half of the dry mixture until a dough forms.
Using a spatula, fold the chocolate chips, toffee bits and walnuts into the dough until evenly distributed.
Divide the dough into 24 balls of cookie dough (weighing around 50g each), and chill them in the refrigerator for 2 - 48 hours depending on the level of fermentation you want to achieve.