Pre-heat your oven to 350°F (180°C) with a rack in the middle. Line a baking sheet with parchment paper.

In a small pot, combine the 2 tablespoons of sugar with 1 tablespoon of water and the 4 tablespoons of amaretto. Bring to a slight simmer and stir for about 1 minute until the sugar has dissolved. Remove from the heat and let cool.

In a large mixing bowl, whisk together the sugar, salt and soft butter until homogeneous and frothy. Whisk in the eggs, one by one. Finally, whisk in the ground almond, almond extract, lemon zest (optional) and mix until smooth.

With a sharp serrated knife, slice each croissant in half, like you would for a sandwich. Place on the prepared baking sheet.

Spread about two tablespoons of the almond cream on the bottom half of each croissant. Place the top halves over top and press lightly to seal.

Brush the top of the croissants generously with the syrup and sprinkle with sliced almonds.

Bake for 13-15 minutes, or until the croissants are golden-brown. Some almond cream may ooze out slightly, this is normal. Carefully transfer the croissants to a wire rack and let cool for 5 to 10 minutes.

Dust with powdered sugar just before serving.