Heat the oven to 160 C / 320 F. Spread the hazelnuts on a 33 x 46 cm / 13 x 18 inch sheet pan and toast for 10 minutes, until the skins split and the nuts smell fragrant. Wrap them in a clean towel, rub off most of the skins, and cool for 10 minutes.

Pulse the cooled hazelnuts with 2 tbsp of the powdered sugar in a food processor for 20 to 30 seconds, until they look like fine, dry sand.

In a 26 cm / 10-inch bowl, beat the butter and the remaining powdered sugar with a hand mixer for 2 minutes, until pale. Beat in the yolk and vanilla, then stir in the ground hazelnuts, flour, cocoa, and salt until no dry streaks remain. Press the dough into a disk, wrap it, and chill for 30 minutes.

Heat the oven back to 160 C / 320 F and line two 33 x 46 cm / 13 x 18 inch sheet pans with parchment. Roll the dough into 10 g balls, about 2 tsp each, to make 60, and set 30 on each pan, 5 cm apart. Bake on the middle and upper-middle racks for 14 minutes, swapping and rotating the pans at 7 minutes, until the tops look matte and the edges feel set. Cocoa dough hides browning, so go by the clock and the edges, because cookies baked past 16 minutes turn bitter.

While the cookies bake, bring the cream to a bare simmer in a 1-quart / 1 L saucepan. Pour it over the chopped chocolate in a 20 cm / 8-inch heatproof bowl, wait 1 minute, then stir until smooth and glossy. Leave it at room temperature for 30 minutes, stirring now and then, until it thickens to the texture of soft peanut butter.

Let the baked cookies cool on the pans for 20 minutes, then pair them by size. Turn one half of each pair upside down, spoon a generous 1 tsp (about 7 g) of ganache onto the flat side, and press the matching cookie on top with a slight twist. Store in an airtight container in the fridge.