1 batch croissant dough, through the third fold and chilled overnight (see Butter croissants)
100 g shelled unsalted pistachios (3/4 cup)
60 g sugar (1/3 cup)
60 g soft unsalted butter (4 tbsp)
1 egg
20 g flour (2 tbsp)
1/4 tsp almond extract
100 g dark chocolate (70%), chopped, or chips
1 egg beaten with 1 tbsp milk
How to make it
Pistachio cream (10 min): Blitz pistachios with sugar in a food processor until fine and starting to clump. Add the butter, egg, flour and almond extract and blend smooth. Chill while you roll.
Roll (10 min): Roll the dough to a 30 x 45 cm (12 x 18 in) rectangle, 4 to 5 mm thick, long side toward you.
Fill (5 min): Spread the pistachio cream to the edges, leaving a 2 cm strip bare along the far side. Scatter the chocolate over it and press it in lightly.
Roll up and chill (35 min): Brush the bare strip with egg wash. Roll up tightly from the near side into a log and pinch the seam. Wrap and freeze 30 minutes so it slices cleanly.
Slice and proof (2 hr): Cut the log into 3 cm (1 1/4 in) slices with a sharp knife. Lay them flat on lined trays, 8 cm apart, and tuck the loose tail under so it doesn't unroll. Proof at warm room temperature until puffy, about 2 hours.
Bake (20 to 22 min): Heat the oven to 190C (375F). Brush the tops with egg wash. Bake 20 to 22 minutes until deep golden and the bottoms are browned. Cool 10 minutes.
Tips
Tucking the tail under is what keeps the snail shape.
Chop the chocolate rather than using chips if you want it to melt into streaks.