1. Prepare the glaze by heating 120 ml water, 1 tablespoon caster sugar, and 1 teaspoon vanilla extract in a saucepan until boiling. Set aside to cool.
2. For the frangipane filling, beat 60 grams of softened unsalted butter and 100 grams caster sugar together until pale and fluffy.
3. Add 1 small egg, 90 grams ground almonds, 1/2 teaspoon salt, and 50 grams pistachio paste to the butter mixture and mix well.
4. Preheat your oven to 180°C (350°F).
5. Slice each of the 6 croissants horizontally, leaving one side intact.
6. Dip the sliced croissants into the sugar syrup glaze, allowing excess to drip off, and place them on a lined baking tray.
7. Fill each croissant with the pistachio frangipane and a portion of the 100 grams chopped dark chocolate.
8. Close the croissants and spread additional pistachio filling on top, then sprinkle with 50 grams of chopped pistachios.
9. Bake in the preheated oven for 15-20 minutes, until the filling is set and the croissants are golden brown.
10. Allow croissants to cool slightly before serving them warm.