Preheat oven to 350°F degrees. In a saucepan, mix together sugar and water, bringing it to a boil. Add vanilla extract and orange juice. Remove from the heat.
Slice croissants in half lengthwise. Using a pastry brush, soak croissants with syrup on both sides. Note: Croissants will be wet, but will dry out during baking.
Melt dark chocolate chips in the microwave in 30 second intervals, stirring in between each. Once chocolate chips are completely melted, stir in the Nutella until smooth.
Spread a spoonful of chocolate filling into each croissant bottom (optional: sprinkle on crushed nuts). Cover with the croissant top. Use a pastry brush to spread syrup on the top of each croissant. Repeat until all croissants are filled.
Bake for 25 minutes. Sprinkle with powdered sugar and serve!