Combine all filling ingredients in a bowl until it forms a thick, spreadable paste. Adjust the cream or milk to get a consistency that’s easy to spread without being runny.
Preheat your oven to 350°F (175°C).
Carefully slice each croissant open and spread a generous amount of filling inside. Press the edges lightly to secure the filling.
Brush the tops of the filled croissants with the egg yolk and sprinkle with crushed nuts and sugar if desired.
Place the croissants on a baking sheet lined with parchment paper. Bake in the preheated oven for about 8-10 minutes, or until the filling is warm and the croissants are lightly toasted.
Dust with powdered sugar just before serving for an extra touch of sweetness.
