Preheat your oven to 160C (320F). Prepare an 8" x 12" slice pan by lining with parchment paper. Set this aside for the moment.

Start by adding the melted butter to a medium sized mixing bowl (I generally just melt the butter in the microwave because it's the easiest way).To the melted butter, add the brown sugar, desiccated coconut, all purpose flour and sourdough starter. Mix together using a spatula. If the mixture is a bit stiff, I just use my hands to mush it together.

Tip the base mixture into the pan you prepared earlier and use your fingers to push the base mixture out until it covers the whole base of the pan. If you need to, use the base of a measuring cup or glass to flatten it out. You want to get it fairly even but it will flatten out a bit while it cooks.

Place the base into the oven to cook at 160C (320F) for 10 minutes while you prepare the filling.

To a medium saucepan, add the cubed butter, brown sugar, vanilla and salt if you're adding it. Gently melt the butter over a medium heat and whisk until it's all combined with the sugar and vanilla extract.

Once the mixture starts bubbling around the edges, pour in the can of condensed milk and whisk into the mixture keeping it over a medium heat.

Once the mixture is fully combined, allow it to warm through over medium heat, whisking fairly often to stop it catching on the bottom. You'll see that it will start to bubble quite a bit after a few minutes.

Once it starts to thicken and bubble, whisk the mixture continuously over a medium heat for around 4 to 5 minutes. If you feel it's getting too hot, turn the heat down, but as long as you keep whisking it, it should be fine.

After 5 minutes of continuous whisking over medium heat, your caramel should be thick and glossy and darker in color. Remove from the heat. This should coincide with the base being baked through.

Once the base has cooked, carefully remove it from the oven. Drop the oven temperature to 150C (300F).

Carefully pour the hot caramel over the base, using a spatula to spread the caramel fairly evenly.

Return the slice to the oven and cook for 10 minutes at 150C (300F). The caramel will bubble and that's ok. Let it cook.

Once 10 minutes is up, remove the sourdough caramel slice from the oven and allow it to sit and cool for 10 minutes before placing into the fridge (ensuring that you have it sitting on a board or something to protect the shelf in your fridge).

Once the caramel is set (usually takes around 2 hours) melt the chocolate. I put the chocolate chips in a bowl and microwave in 30 second increments until liquid (stirring in between each blast) but you can do this with a double boiler if you prefer.Once the chocolate is melted, add the vegetable oil (if using) and stir well to combine.Pour melted chocolate over the top of the caramel, using an offset spatula to spread it all the way to the edges. You need to work quickly as the caramel will harden the chocolate quite fast.

Return the slice to the fridge for 10 minutes for the chocolate to fully set.