Method

• 1.
  For the dulce de leche, place all ingredients in a large heavy-based saucepan over medium heat, stirring until sugar dissolves. Increase heat to high and bring almost to the boil. Reduce heat and simmer, stirring very occasionally, for 1 hour or until mixture reduces and becomes light beige.
• 2.
  Place a small plate in the freezer. Cook dulce de leche, stirring often, for a further 20 minutes or until a deep caramel. To check consistency, spread a little on the cold plate and pass a spoon through the centre. If mixture does not run, it’s ready. Set aside to cool, then store in a clean jar.
• 3.
  For the shortbread, place butter, sugar, vanilla and a pinch of fine salt in a stand mixer fitted with the paddle attachment. Beat until light and fluffy. Sift over flour, cornflour and cocoa. Beat on low speed until all dry ingredients are incorporated. Shape into 2 discs and enclose in plastic wrap. Refrigerate for 30 minutes.
• 4.
  Preheat oven to 160°C/140°C fan-forced. Line 2 baking trays with baking paper. Roll dough between 2 sheets of baking paper lightly dusted with flour until 5mm thick. Chill dough for 15 minutes. Using a 6.5cm round cutter and dipping into extra cocoa between each cut, cut 16 rounds from dough, re-rolling and cutting dough scraps. Place rounds on prepared trays, 2cm apart. Chill again until firm. Bake for 12-15 minutes until firm to the touch. Cool on trays.
• 5.
  Spread half the cookies with a little dulce de leche. Sprinkle with salt flakes, then sandwich with remaining cookies.