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PRE-HEAT your oven to a temperature of 180 degrees celsius or 356 Fahrenheit. Lightly grease the baking tin with some butter, and set aside. MIX 250 gram plain flour with 75 gram caster sugar in a bowl using your hands.
RUB in 175 gram softened butter until the mixture looks like fine breadcrumbs. Keep kneading the mixture until a dough forms, then press the dough into the base of the prepared baking tin. POKE some holes in the dough using the fork.
PLACE the baking tin in the oven for 20 minutes, or until it's firm to the touch and slightly browned. Remove from the oven and let cool off to room temperature.
MEASURE 100 gram butter, 100 gram sugar and put it in a small saucepan along with the condensed milk. HEAT very gently until the sugar has dissolved completely. BRING to a boil by turning the heat up, make sure to continue stirring while its boiling!
TURN down the heat to medium-low and let simmer for about 5 minutes, or until the mixture has somewhat been thickened. POUR the caramel over the shortbread and let rest cool off to room temperature.
LASTLY, make the topping. ADD water to the slightly larger pan, let come to a boil. MELT 300 gram dark chocolate in a bowl by placing the bowl inside the bigger pan.
This French technique is called au bain-marie. It prevents the chocolate from burning by slowly melting it. When the chocolate has melted completely, pour over the cold caramel and leave to cool off yet again.
Serve the millionaires shortcake by cutting it into equal squares or bars. Enjoy!