In a medium sized mixing bowl, add the butter, sugar and egg and whisk until light and creamy.
Add the sourdough starter (or discard) to the butter mixture and continue to whisk until fluffy.
On top of this, add the all purpose flour, baking powder and raisins. Gently mix with a spatula until the mixture is well combined.
Cover the bowl and place into the fridge for around 2 hours (the mixture can remain in the fridge for up to 48 hours).
When You're Ready to Bake
Preheat the oven to 180C (350F) and line a large baking tray with parchment paper.
Remove the cookie dough from the fridge and allow to stand at room temp for around 10 minutes.
Use a cookie scoop to make 8 equal balls of cookie dough. Roll each ball into the cornflakes and then place onto the pre prepared baking tray.I've made 8 large cookies but you can make smaller ones if you prefer (you'll get approx. 14 to 16 smaller cookies from a single batch of this cookie dough).
Bake the sourdough cookies for around 15 minutes at 180C (350F). Cookies are done when they are golden brown but still fairly soft in the middle.
Remove the tray from the oven and allow the cookies to cool on the tray for around 10 minutes. Once they've firmed up, transfer to a wire rack to cool completely.
Notes
These sourdough cornflake raisin cookies are super easy to make (and a great sourdough baking project to take on with kids in the kitchen) but here are a few tips and tricks to ensure you have success first time and every time:
Wet cookie dough - this cookie dough is quite soft, but firms up nicely in the fridge. Don't be tempted to add more flour to the dough or else you'll get very cakey cookies.
Sourdough Discard - if using your discard, you don't want it to be too runny as this will make your dough wetter than it needs to be (and you don't want to add more flour for the reasons above). I recommend using fresher discard, so not more than about 24 hours old, for this recipe.
Don't over bake the cookies - While you want crunchy, golden exteriors, you want the middle to be buttery and soft. Allow the cookies to bake until the edges are golden but the middles are still soft. The cookies will firm up once cooled.