No need to preheat the oven yet, this dough needs to chill.

Place the flours, bicarb and chopped double decker bars into a medium sized bowl and mix to combine.

Place the butter and sugars into a large bowl or the bowl of your stand mixer and beat until light and fluffy, about 2 minutes on med-high speed.

Add in the eggs and vanilla.

Mix until well incorporated and smooth, about 1 minute on med-high speed.

Add in the flour mix and mix on low until a dough forms.

Tip the dough into a medium sized bowl (the one you measured the flour in is fine) and cover with cling film.

Place in the fridge for at least 6 hours, overnight if possible. Once chilled, preheat the oven to 180 degrees C/350F and line 2-3 baking trays.

Using a 1.5tbsp cookie scoop, place balls of dough 2 inches apart on your baking trays. I usually get 12 cookies per tray.

Flatten the dough balls very slightly so they don't fall off the tray and place in the oven for 8-9 minutes. Cookies are done when ever so slightly brown around the edges and still soft and underdone in the middle. The cookies will firm up during cooling, but you really don't want to over bake them as they won't be as good!

Cool on trays for at least 10 minutes before transferring to a wire rack to cool completely.

Cookies will keep in an airtight container, at room temperature for 5 days.