Preheat the oven to 220°C and line three baking trays with greaseproof paper.
Mash butter and lard with a fork until creamy and mark into quarters.
Rub one quarter of the fat into flour and salt, then mix with vinegar and cold water to form a dough.
Roll the pastry on a floured surface to a 1cm thickness.
Dot a quarter of the fat over the pastry, fold into thirds, and repeat the rolling process three times.
Chill the wrapped pastry in the fridge for 30 minutes.
Dissolve brown sugar and butter on the hob, add condensed milk, boil briefly, and cool.
Roll pastry to 3mm thickness and cut into thirty-six 13cm x 8cm rectangles.
Prick 12 rectangles with a fork, dredge with icing sugar, and freeze for 5 minutes.
Bake frozen rectangles for 10-12 minutes at 220°C, then weight down with another tray and bake for 7-10 minutes more.
Bake the remaining 24 rectangles in two batches following the same method.
Whip double cream to stiff peaks, fold in liqueur and icing sugar, then chill in a piping bag.
Transfer cooled caramel to a bottle and chill.
Mix cocoa, icing sugar, and boiling water into a stiff paste.
Spread icing over 12 rectangles, add caramel lines, and feather with a toothpick.
Assemble by layering two un-iced rectangles with cream and caramel before topping with an iced rectangle.