This is a typically Scottish treat in that it has far more sugar than is at all necessary! It's delicious but very rich so make sure to cut it into small squares.
The key to Millionaire's Shortbread is in the thickness of the layers. You want a decent sized layer of quite dry shortbread on the bottom to give structure, followed by a layer of caramel twice the height of the base. Then a thin layer of chocolate on top - not too thick or the caramel will squidge out the sides when you bite into it.
225g plain flour
75g caster sugar
375g salted butter
1 (379g) can sweetened condensed milk
4 tablespoons golden syrup
250g chocolate ( I usually use 1/2 milk and 1/2 dark so the chocolate isn't too sweet)
Preheat oven to 170C.
Put the flour and sugar into a mixing bowl and rub in 175g of butter, until the mixture looks like damp sand.
Press the shortbread mixture down firmly into the base of a buttered 8 x 8 square cake tin and make sure it's even. Prick the surface with a fork and cook for five minutes, then lower the temperature to 150C, and cook for a further 30 minutes until pale golden and no longer doughy.
Allow the crust to cool in the tin.
Melt the remaining butter in a heavy bottomed pan, then add the condensed milk and golden syrup. Whisk the mixture well until the butter is thoroughly incorporated.
Heat on medium 6-7 minutes until boiling, stirring constantly. The mixture may bubble up when it's boiling, just lift the pan off the heat until it goes down. The caramel is ready when it's thickened enough to coat the back of a spoon and turned a light golden brown.
Pour the caramel over the shortbread and leave to cool at room temperature and then place in the fridge for 1 hr.
Melt chocolate and pour over the set and chilled caramel mixture and leave to cool again.
Once set, cut the caramel shortbread into squares. You should get at least 24 from this.