1. Preheat your oven to 180°C (or 160°C if you're using a fan oven).

2. Line an 8x10-inch rectangular tin with baking paper. Let the paper hang over the edges to allow for easy removal later on.

3. In a large bowl, mix the flour and sugar together. Rub the margarine into the flour and sugar using your fingers, then press everything together in your hands to form a dough. If the dough is too wet, add more flour and if it's too dry, add a small amount of margarine.

4. Press the dough out evenly into your tin, making sure it gets all the way to the edges. Use a fork to create small holes all over the base, which will allow air to escape while it's baking.

5. Bake for around 20 minutes, until the corners are very lightly golden. Allow it to cool completely in the tin.

For the caramel layer:

1. Add the condensed milk, margarine, brown sugar and vanilla into a large non-stick pot. Turn the heat to low and gently stir every so often until the margarine has completely melted.

2. Once all of the ingredients have melted together, turn the heat up to high. Stir continuously until the mixture reaches 118°C. I recommend using a candy/jam thermometer for best results but you can also tell that it's ready when it begins to thicken slightly and pull away from the edges of the pot.

3. Pour the caramel over your cooled shortbread and gently spread it out to the edges. Let it sit at room temperature for 30 minutes.

For the chocolate layer:

1. Melt the chocolate in a pot over a low heat then pour it over your caramel shortbread. Gently spread it out to the edges, being careful not to smear it into the caramel.

2. Place it in the fridge for 2-3 hours until the chocolate layer has completely set, then cut into pieces and enjoy!