Preparation

• Unfold the puff pastry and press any creases flat with your hands. Lightly dust a work surface with the all-purpose flour to prevent sticking. Using a 2-inch cutter, cut 40 circles from the pastry. From 20 of these circles, cut out 1-inch holes in the center using a smaller cutter or knife—these will become the rim rings that sit on top. Keep the 20 whole circles as your bases. Brush the beaten egg generously over all pastry pieces, ensuring even coverage for even browning. Place the 20 whole circles on a parchment-lined baking sheet, then carefully stack a holed circle on top of each one, pressing gently to adhere. This creates a double-layer structure that will puff up during baking and create natural cups.
• Refrigerate the assembled pastry stacks for 30 minutes—this resting period allows the gluten to relax and helps the pastry puff evenly without puffing unevenly or shrinking. Preheat your oven to 400°F while the pastry chills. Once chilled, bake the stacks for 16-20 minutes until they're golden brown and have puffed up significantly. The pastry should be crispy and the centers should have risen to create natural cup-like indentations. Remove from the oven and let cool on the baking sheet for 5-10 minutes until they're cool enough to handle but still warm, which makes removing the center tops easier.
• While the pastry cools, prepare the chicken salad by combining the diced cooked chicken, minced red onion, chopped toasted walnuts, and quartered grapes in a medium bowl. In a separate small bowl, whisk together the Greek yogurt, Dijon mustard, sea salt, and freshly ground black pepper until smooth and well combined. Taste the yogurt mixture at this point and adjust the seasonings—sometimes I add an extra pinch of salt if the chicken is mild. Pour the yogurt mixture over the chicken mixture and fold gently until all ingredients are evenly coated and the salad comes together. Fold in the fresh chives just before filling to keep them bright and fresh.
• Once the pastry cups have cooled slightly, use a small spoon or melon baller to gently remove the center circle from the top of each pastry cup, creating an opening for the filling. Spoon approximately 1 tablespoon of the chicken salad mixture into each pastry cup, mounding it slightly. Fill these just before serving so the pastry stays crispy and doesn't get soggy from the yogurt-based filling.