To make the shortbread base

Cup of Yum
1. Preheat the oven to 180C/350F.
2. Prepare a 23cm x 33cm (9" x 13") baking dish by lining it with parchment paper (baking paper), leaving a slight overhang, which will make it easier to lift the slice from the tin later.A helpful tip: Lightly coat the tin with butter or cooking spray before laying the paper to help it adhere more effectively.
3. Add the unsalted butter, sugar and vanilla extract to a large mixing bowl or the bowl of a stand mixer.
4. Beat the ingredients together for a minimum of 5 minutes until the mixture turns light and fluffy. An electric hand mixer or a stand mixer is ideal for this step.
5. Add the flour, baking powder and salt to the butter mixture. Beat together on a low-medium speed until the mixture resembles fine crumbs.
6. Use your hands to lightly press and bring the crumbs together until they form a uniform dough.
7. Set aside roughly 220g of the shortbread dough in a separate bowl. This will be used for the crumb layer later.
8. Break the rest of the dough into rough chunks and evenly spread it across the base of the baking tin. Press the dough into the base of the dish. Use a spatula or the base of a glass to flatten and even out the surface.
9. Prick the base all over with the prongs of a fork.
10. Bake the biscuit base in the preheated oven for 15 minutes or until it turns a very light, golden brown colour.

To make the caramel layer

1. While the biscuit base is baking, add the condensed milk, butter, dark brown sugar and golden syrup to a small saucepan.
2. Place the saucepan over low heat and gently warm the mixture until the butter is fully melted and it starts to come to a slow, gentle boil, stirring constantly.
3. Once the mixture comes to a slow, gentle boil, continue cooking the caramel for about 5 minutes, maintaining low heat and constantly stirring. You'll notice it slightly thickening and taking on a slightly darker colour.**To check the consistency of the caramel, lift some of the mixture and drizzle it over the surface. It should form a trail that vanishes within 2-3 seconds. **You can also check the consistency by coating the back of a metal spoon. The surface will slightly dull after a second or two, and you should be able to draw a clear line through it. There may still be minimal dripping, but it will quickly stop.
4. Remove the caramel from the heat and stir in the teaspoon of salt and vanilla extract.
5. Immediately pour the hot caramel over the partially baked biscuit base. Tilt the pan from side to side until the caramel evenly coats the entire surface.
6. Revisit the dough you set aside earlier and crumble small pieces across the surface of the caramel, leaving some gaps for the caramel to remain visible.
7. Bake for an additional 20 minutes, or until the caramel filling turns a rich, golden brown and the crumb topping becomes light brown.
8. Allow the bake to cool completely before lifting it from the tray and slicing it into squares. Sprinkle over the optional coarse sea salt. A helpful tip - Use a large, sharp knife to slice the tan squares. The outer edges may be darker and chewier, which some people enjoy the most. If you prefer a neater appearance, trim the sides before cutting them into neat squares or bars.