[Preparation]
Add hot water to coffee and stir to dissolve. If it doesn't dissolve, microwave it for about 10 seconds. Let it cool well with ice water.
Cut well-chilled unsalted butter into 0.5-1 cm / 0.2-0.4 inch cubes.
Chop roughly nuts.

[Shortbread]
Put diced butter, cane sugar, bread flour, cake flour, and salt in a large bowl.
Use your hands to crush and rub the butter cubes until the mixture turns yellowish and looks like dry crumbs. (quickly so that the butter doesn't melt)

Pour in the coffee while drawing circles. Use a spatula to fold it until looks like dry crumbs. Add nuts and fold.

Use your hands to press it until comes together roughly. Put it out on the work surface lined with plastic wrap. Use your hands to press it until comes together, and flatten it.

Fold it in half and press. Repeat ''folding in half and pressing'' 10 times until smooth.

Sandwich the dough between cake films (or plastic wrap), and roll it out to 1 cm / 0.4'' thick. Let it sit in a freezer for 15 minutes, leaving the films on.

[Cut out, Bake]
15 mins later, use a round cookie cutter, 5 cm / 2 inches in diameter, to cut it out. 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 21 minutes. Remove from the oven, and let it cool as it is for about 15 minutes. Transfer to a rack to cool completely. After cooling, store them in an airtight container at room temperature.