Preheat the oven to 410 F (210 C).Prepare 2 baking sheets by lining them with parchment paper. Set aside.Shell the langoustine tails and set them aside in the refrigerator.Peel, wash, and dry the spinach.Feuillantines pastry: Mix the flour with the egg whites, add a pinch of salt and a pinch of powdered sugar, then incorporate the melted butter. Mix everything together and let it cool.Place 8 portions of pastry on the prepared baking sheets, spacing them out well (4 feuillantines per sheet), then use a brush to spread the pastry thinly into a disk shape.Sprinkle with sesame seeds, then bake for 8 minutes or until the feuillantines are golden brown.Remove them from the oven and set them aside.Creamy curry sauce: Melt the butter in a saucepan, add the curry powder and cook for 1 minute, then add the cream and bring to a boil.Cook for 5 minutes over low heat, adjust the seasoning, then strain through a fine-mesh sieve.Set aside and keep warm.Langoustines: Season the langoustine tails with salt and sprinkle with curry powder.Then sauté them quickly in olive oil, 1 minute on each side.Drain them on paper towels.Spinach: Melt the butter in a saucepan over high heat until it turns a nice brown color.Add the spinach and stir quickly. Stop cooking after 2 minutes to preserve its bright color and crunch.Assembly: Divide 3/4 of the spinach among the bottom of each warm plate.Cover with a feuillantine, then add some spinach and cover with langoustines.Top with a second feuillantine.Drizzle with creamy curry sauce and serve immediately.