In a bowl, mix the flour and almonds. Reserve.
Beat the butter on medium-low speed. Add both powder and white sugar and salt. Mix. Add the vanilla and the egg yolk and beat for 1 more minute.
Add the flour-almond mixture in batches. Turn onto the countertop and shape the dough into a ball. This is a soft dough, but you can add 1 tbsp extra flour if needed.
Roll out the dough between two sheets of parchment paper and chill for at least an hour.
Preheat the oven to 170°C/340°F. Cut out the cookies with a cookie cutter and place them apart on a greased and floured baking sheet (or lined with parchment paper).
Bake for 15 minutes, or until they are slightly browned around the edges. Let them cool, fill them with dulce de leche and form the sandwiches.