In a medium bowl sift together flour, cornstarch and baking powder. Whisk to combine.
In a separate bowl, cream together softened butter with powdered sugar until light and fluffy. Butter should almost look white coloured.
Add egg yolks one at a time to the butter mixture. Mix well after each addition.
Add the rum and vanilla extract.
Fold in the flour mixuture into the butter mixture. Mix until just combined. It should come together as a dough.
On a lightly floured surface, knead dough just until all the flour mixture is combined and dough is smooth.
Wrap dough in cling film and refridgerate for 1 hour.
Preheat oven to 300 degrees F.
Roll out dough on a lightly floured sheet to 2-3mm thick. Cut with a cookie cutter of your choice and place onto a parchment lined baking sheet.
Bake for 10-12 minutes or just until they begin to brown on the bottom.
Let cool and fill with dulce the leche, then sandwich two cookies together. Sprinkle with powdered sugar to serve!