Grease baking sheets and line them with parchment paper. Draw 1 1/2-inch (4 cm) circles on the paper, spacing them about 1 inch (2.5 cm) apart. Preheat the oven to 300°F (150°C).

Blend the powdered sugar and hazelnut flour in a food processor until very fine. Sift the mixture and discard any large pieces left in the sieve.

Mix the instant coffee and boiling water together in a small bowl.

Beat the egg whites with a stand mixer or hand mixer until foamy, then gradually add the granulated sugar. Continue beating until stiff peaks form. Gently fold in the powdered sugar and hazelnut mixture, along with the coffee mixture, in several additions. Stir gently. Use a spatula to fold the batter by scraping it against the sides of the bowl. Be careful not to overmix, so the batter doesn't become too runny.

Pipe the macaron batter onto the prepared circles. Tap the baking sheets on the counter to release air bubbles. Let the macarons rest for 30 minutes to form a skin. Bake for 20 to 30 minutes at 300°F (150°C). Remove the baking sheets from the oven and wait until the macarons can be easily lifted off the parchment before removing them. The shells can be stored in a dry place for a while if not filled.

For the ganache: Bring the heavy cream to a boil. Once boiling, add the white chocolate and stir until completely smooth. Refrigerate. It's best to prepare this the day before assembling the macarons.

Assemble the macarons by piping the ganache onto one shell and sandwiching with another. Let the filled macarons rest in the refrigerator overnight. They taste best the next day.