Preheat the oven to 180°C (350°F) and line two baking trays with baking paper.
In a large bowl, mix together the butter, sugar, vanilla extract, egg and egg yolk until creamy and pale in colour.
Sift in the cornflour, plain flour and baking powder and mix together until the dough comes together.
Divide the dough into two balls, wrap in clingfilm and refrigerate for at least 30 minutes.
On a lightly floured work surface, roll out one of the balls to a thickness of around 5mm (0.2 inches).
Using a round cookie cutter (or the rim of a glass), cut circles of dough out and place onto one of the baking trays.
With the remaining dough, re-roll and repeat until all the dough is used up.
Bake for 12 minutes or until golden brown, then remove from oven and leave to cool completely on a wire rack.
Once the biscuits have cooled, spread a teaspoon of dulce de leche onto half of them.
And sandwich with remaining biscuits.
Roll in desiccated coconut to finish off.