1. In a bowl whisk the flour, cornflour, bicarb and baking powder.
2. In another bowl, beat the butter and sugar with an electric mixer until light and creamy. Add the egg yolks one at a time, then add the vanilla and milk.
3. Gently fold in the flour mix to make a crumbly dough. Wrap in cling film and chill for 1 hour. Preheat oven to 180°C (Gas Mark 4).
4. On a lightly floured surface, roll the dough to 5mm thick and cut with a biscuit cutter. The dough will crumble so do not worry if you have to re-roll. Continue until all the dough is used up.
5. Line a baking tray with non-stick baking paper and place the biscuits on it, 1 cm apart. Bake for 9-10 minutes, then cool on a wire rack.
6. Spread the underside of a cooled biscuit with a teaspoon of dulce de leche, then sandwich together with another biscuit until the caramel oozes out the sides. Finally, sprinkle with icing sugar and serve.
Recipe courtesy of Kenwood's Around the World in 80 Plates