Preheat the oven to 350°F (180°C). Prepare 2 baking sheets lined with parchment paper.
On a medium plate, combine equal amounts of flour, superfine/caster sugar (or granulated sugar), and powdered sugar (yes, there should be all these 3 ingredients) - you can just eyeball the amounts. You will use this for sprinkling the dough while rolling it out (don´t use just flour!).
Divide the dough into two parts, leave one part in the fridge.
Sprinkle some of the flour-sugar mixture on a silicone rolling mat (best for rolling out cookie dough or anything, really). Place one part of the dough on a rolling mat and roll out very thinly (about 1/8 inch or 3-4mm), sprinkling the dough with the flour-sugar mixture if it´s sticking to the rolling mat.
Cut the dough into 2-inch (5cm) squares using a knife/ravioli cutter wheel/pizza knife.
Place some of the filling (about 1 teaspoon) in the middle of each pastry square (don´t add too much filling).
Fold over the opposite corners of each square and seal well: dip your finger in water and wet one corner of the pastry square. Place the oposite corner of the square on top, moisten it with water and press well to combine both parts of the dough (this is important, otherwise, the cookies will open up in the oven). All of this is shown on the photos in the body of the post or on the video (scroll up to see it).
Place each cookie on the baking sheet lined with parchment paper. The cookies need to be just a little bit spaced apart - they won´t rise much in the oven.
Repeat with the remaining dough.