Pour liqueur into an 8-inch square pan or rimmed plate that can fit a whole matzo square. Working with one matzo square at a time, press matzo gently into liqueur and let stand 1 minute, flipping once halfway during soaking time. Set squares aside on a plate.

Pour remaining soaking liqueur into a liquid measuring cup. (You should have about 6 ounces; if not, add additional liqueur to equal 6 ounces in measuring cup.).

Place chocolate, sugar, and 6 ounces liqueur in a double boiler or a bowl placed on top of a steaming pot of water. Whisk over medium-low until chocolate is melted and mixture is smooth, about 3 minutes; turn off heat.

Add egg yolks, one at a time, to double boiler or bowl, whisking constantly until combined. Turn heat to medium-low and whisk constantly for 3 minutes or until mixture has reached 160°F over simmering water.

Remove from heat and whisk in butter, 1 tablespoon at a time, until smooth and emulsified.

Place 1 soaked matzo square on a plate; cover with a thin layer of chocolate mixture (about ¼ cup) and sprinkle evenly with about ¼ cup of finely chopped hazelnuts. Continue to layer matzo squares, chocolate, and hazelnuts (four more layers), spreading chocolate over edges to coat sides of cake and making sure top layer of chocolate is smooth. Use any additional chocolate to coat any gaps on the sides. Sprinkle sea salt and remaining hazelnuts over top.

Place cake in refrigerator, uncovered, until chocolate is thick and cake is set, about 2 hours. Remove from refrigerator about 1 hour before serving.