SHORTBREAD BASE

1. Lightly grease an 8” (20cm)  shallow square baking pan, then line with parchment paper. Put the butter into a large bowl and beat well with a wooden spoon or an electric mixer until creamy and pale. Add the sugar, salt, and vanilla and beat again until even paler. 
2. Sift the flour over the creamed butter and sugar. Using a spatula, gently work the flour into the mixture to make an even dough that starts to clump together. 
3. Press the dough into the prepared pan, then level and smooth it with the back of a spoon. Prick it all over with a fork, then chill for 10 minutes, or longer if you like, until firm. Meanwhile, preheat the oven to 325°f /160°c.
4. Bake for 25-30 minutes, or until the shortbread is golden all over. Let cool completely. 

CARAMEL

1. Melt the butter, sugar, syrup, and salt together gently in a saucepan, then stir in the condensed milk. 
2. Bring the caramel to a simmer, stirring constantly with a spatula, and let it bubble for 4 minutes, or until it thickens and smells like creamy toffee. It should be thick enough for the spatula to leave a trail in the caramel for a few seconds. Don’t leave the pan or stop stirring during this step, because it can easily burn on the bottom. 
3. Pour the caramel over the shortbread, then let it cool completely. 

CHOCOLATE TOPPING

1. Melt the chocolate either over a saucepan of water or in the microwave. Stir in the oil, then pour this over the caramel. Sprinkle with the salt and let set at room temperature, or in the fridge if it is a hot day. The oil helps stop the chocolate setting too hard, which can make it difficult to cut. 
2. When the chocolate is just set, mark it into squares, then chill until completely firm.  
3. Cut into cubes to serve. For a really clean finish, wipe the blade of your knife with a slightly damp cloth between each slice. Store in an airtight container for up to 3 days.