Weigh the flour into a medium mixing bowl. Add the baking soda, xanthan gum and salt and whisk to combine.

Weigh the sugars into a medium heat proof bowl (I use a steel bowl). Whisk to combine.

Mash your banana and set aside.

Place the butter in a medium heavy bottomed saucepan over a medium heat. Cook, stirring regularly, until it starts to foam and pieces of brown butter start floating up through the foam. It should smell beautifully nutty and have a nice deep brown colour once the foam subsides.

Weigh the browned butter into the bowl with the sugar. It should weigh 100g or very close to that number.

Stir to combine, then stir through the mashed banana and vanilla if you are using it. Stir in the flour thoroughly until everything is combined. Depending on whether you use xanthan gum or not, the dough should initially look quite firm and glossy. After a few minutes, though, it should relax and create an even layer in the bowl.

Once the dough is cool, cover it and place it in the fridge overnight.

Preheat the oven to 170C and line two large sturdy baking trays.

Use a spoon to dig up the semi firm dough and stir through the chopped dark chocolate. Form the dough into 9-10 cookies, about 1 – 1 ½ tablespoons* (see notes if you are not from Australia) of dough each.

Arrange the cookies with room to spread on each tray. I recommend baking the trays one at a time before best results.

Bake each tray for 12-14 minutes or until the cookies are perfectly spread and cooked through. Repeat with the second tray.

Allow the cookies to cool before eating as they will be crumbly before they set.

Leftover cookies keep well in an airtight container in the fridge.