Shortbread dough

1. In a large bowl, beat softened unsalted butter with powdered sugar with an electric mixer for 2 minutes until creamy.
2. Add the egg and vanilla extract and mix again with an electric mixer to incorporate them.
3. Finish with the dry ingredients, flour, salt and baking powder (optional) and mix with the mixer just until incorporated.
4. Gather the dough with your hands to form a dough ball and cover with plastic wrap before placing in the refrigerator for at least 20 minutes.
5. Roll out the dough to a thickness of 3 mm (use 2 sheets of baking paper to roll out the dough more easily).
6. Using a round fluted cookie cutter about 5 cm in diameter, (a Linzer cookie cutter), cut out rounds of dough and rounds with a star-shaped hole in the center. 

Baking

1. Preheat the oven to 350°F / 180°C and bake the linzer cookies for about 8-10 minutes, until lightly browned on top.
2. Remove the cookies from the oven and let them cool a bit before moving them to a cake rack until they cool completely. (they are still soft when they come out of the oven and will harden as they cool)

Assembly

1. Fill with a jam on the cookies that go underneath, then cover with the cookies on top.
2. Press lightly to spread the jam and sprinkle powdered sugar over all the cookies. (you can also sprinkle the powdered sugar on top of the cookies just before assembling them)