Add all of the cookie dough ingredients to a food processor or stand mixer (paddle attachment). Blitz the ingredients for around 20 seconds or until they form a cohesive, moist cookie dough.You can mix the ingredients by hand if you prefer. I recommend beating the egg, sugar, sourdough starter and butter together first and then fold in the dry ingredients. You're looking for a moist dough that stays together when you roll it into a ball.

Place the cookie dough into a bowl and place into the fridge for around 2 hours (it can stay in the fridge for up to 48 hours though).

Preheat your oven to 190C and line 2 cookie trays with parchment paper (I've used 2 x half sheet pans).

Use a cookie scoop to scoop out balls of dough. I made 24 cookies around 1 tablespoon in size (this equated to 27g of dough per ball).

Place the cookie dough balls onto parchment lined cookie sheets. Using a flat bottomed glass or measuring cup gently press the dough balls flat. You want them to be around 1 cm thick. They will flatten out more when they cook.

Once they're all flattened out, place the trays of dough into the oven at 190C for around 15 minutes. Remove from the oven and allow the cookies to cool on the trays. The cookies will still be soft when you remove them from the oven.

Place the butter, vanilla, salt and powdered sugar into the bowl of your stand mixer and whip using the whisk attachment until very pale in color and light and airy in texture. I whip this filling for at least 2 minutes. It will get lighter in color the longer you whip it.

Spoon the filling into a piping bag and set aside until the cookies are completely cool.

When you're ready to sandwich the sourdough chocolate cookies together, pipe a generous amount of filling into the centre of half the cookies and then sandwich the other half of the cookies on top.