Dulce de Leche: Gently heat the butter and muscovado sugar in a saucepan until the butter has melted and the sugar dissolved. Pour in the condensed milk and then cook over a medium heat, stirring constantly for 4-5 minutes until the mixture begins to thicken, bubbles like toffee and smells of caramel. (Don’t have the heat too high or the condensed milk will burn.) Pour the mixture into a bowl and leave to cool.
Biscuits: Preheat the oven to 180 degrees C. Butter 2 baking trays.
Mix the cornflour, plain flour, ground almonds and baking powder together in a bowl. Cream the butter and sugar together in a second bowl until light and fluffy.
Stir in the lemon zest and vanilla extract then gradually beat in the egg yolks. Add the dry ingredients and mix together with a spoon then squeeze into a dough with hands
Divide into 30 pieces then shape into small balls. Place on the prepared baking trays, leaving a little space between the biscuits, flatten slightly then cook in the preheated oven for 8-10 minutes until just beginning to turn golden around the edges. Transfer to a wire rack and leave to cool.
To Serve: Spread half the biscuits with the cooled dulce de leche, top with the remaining biscuits then dust with sifted icing sugar. Arrange on a serving plate or for a festive feel put into foil cases.
Any remaining dulce de leche can be stored in a sealed plastic container in the fridge for up to one week and served spread on toast for a quick snack.