Argentinian dulce de leche shortbread

To make the dulce de leche:

1. Place all the ingredients (2 L milk, 600 g caster sugar, 1 tsp salt, 1 tsp bicarbonate of soda, 2 tsp vanilla extract) in a large saucepan over a medium heat. Do not stir. Once the milk begins to bubble and foam, remove the saucepan from the heat and skim off the foam. Turn the heat down to its lowest setting and return the saucepan to the stove. 
2. Stir the mixture every 15 minutes and skim off the foam as it rises to the surface. Allow the milk to reduce for up to three hours or until a spoonful of the mixture sticks to the spoon when held upside down. Remove from the heat. 

To make the shortbread:

1. Preheat oven to 180°C. Using an electric hand-mixer, cream the butter and sugar until pale and fluffy. Add the egg yolks and vanilla and whisk into the mixture. 
2. Using the mixer at a low speed, incorporate the cornflour, cake flour, baking powder, bicarbonate of soda and salt in small batches. 
3. Turn the dough out of the bowl and shape into a ball, cover with clingwrap and chill for 30 minutes. 
4. Remove from the fridge and unwrap. Lightly flour a working surface and roll out the dough to a thickness of ½ cm. Using a 7 cm biscuit cutter, cut out rounds of dough and carefully place on a greased baking tray. 
5. Bake for 10–13 minutes, or until the sides of the biscuits begin to brown slightly. Allow to cool and firm. 
6. Spread the dulce de leche over half the biscuits and top with the remaining biscuits to create sandwiches.