[Shortbread]
Cut well-chilled unsalted butter into 0.5-1 cm / 0.2-0.4 inch cubes and put them in a large bowl. Add granulated sugar, bread flour, cake flour, and salt.
Use your hands to crush and rub the butter cubes while mixing (quickly so that the butter doesn't melt) until the mixture turns yellowish, and looks like dry crumbs.

Put it out on the work surface lined with plastic wrap. Use your hands to press it until almost comes together. Fold it in half and press. Repeat ''folding in half and pressing'' 10-13 times until smooth.
[Note]
Please be careful to not melt the butter of the cookie dough. If you melt butter, cookies will spread out and taste greasy.

Sandwich the dough between cake films (or plastic wrap) and roll it out to 8 mm / 0.3'' thick. Let it sit in a freezer for 10 minutes, leaving the films on.

Peel off the upper film, and cut it out with a round cookie cutter, 5 cm / 2 inches in diameter. Arrange them on a baking sheet lined with parchment paper. Re-roll the dough scraps and cut. Arrange them on a baking sheet too.

Let it sit in a freezer for 1 hour.
Preheat the oven to 190℃ / 374 F. Lower the oven to 170℃ / 338 F and bake for 20 minutes. Remove from the oven, and let it cool as it is for about 15 minutes. Transfer to a rack to cool completely.

[Cottage Cheese Cream]
Cream unsalted butter until smooth. Add condensed milk and mix well until smooth. Add cottage cheese and mix well until smooth.

Transfer the cream to a piping bag. Turn over half cookies and pipe the cream over each. Cover with another cookie, and press gently.

Put it in an airtight container and let it sit in a fridge overnight to set the cream.